So you just spent days picking out the perfect car stereo to replace the piece of crap the previous owner had in there.  It finally ships to your house and you’re stoked to get it installed.  You pull out the old head unit and you’re faced with a nasty patch work of wiring from previous botched installs.  What do you do?!

I ran across this scenario countless times while working in the install bay and while it might be annoying, it’s not the end of the world.  In this post, I’m going to teach you how to rewire a car stereo from scratch, clean up that wiring mess, and get it looking back to normal.

 

The Right Tools for the Job

There are a few basic tools you will need to get the job done right.  Since every car is different when it comes to taking the dash apart I’ll skip the tools you’ll need to do that, but below is a list of the tools you will need to get the wiring job done right.  Ironically, most of the tools you need also show up on my list of Top 10 Must-Have Car Electronics Install Tools…go figure.

  • Klein Style Crimpers
  • Wire Strippers
  • Digital Multimeter
  • Soldering Iron (Optional): If you don’t have one or you don’t feel comfortable soldering in the dash, you can make your connections with butt connectors.  Just make sure you use good quality ones like these.
  • Speaker Tester (Optional): If you don’t have one I’ll provide a link to where you can get one here.  You can also use a 9-volt battery, I’ll explain how a little later.
  • Tesa Tape (Optional): This helps with wire management, make your wire harness look neater, and helps prevent the wire harness from rattling in the dash.  Here is a link to where you can find some.

Prep Work

If you are reading this post, then you are probably faced with a wiring harness that looks something like this:

Bad-Wiring

Preparing the Factory Harness

You’ll want to start off by removing the wiring harness from the old head unit.  Cut each wire behind any connectors so that only the factory wire is left.  Make sure you cut all the factory wires to the same length so your final result will have a clean look.

**Important Note**

When you are cutting the wires in the factory harness, only cut one wire at a time.  If you try cutting multiple wires at the same time, you may end up shorting your constant power circuit to ground and blow the fuse.  If this does happen, it’s an easy fix to replace the fuse but it will make things more difficult later on when we get to the point of testing the wires.

 

Preparing the New Radio Harness

Just like with the factory harness, cut all the wires on your new radio harness to the same length.  If you will be using butt connectors to make your connections, crimp one onto the end of each wire on the radio harness.

 

Aftermarket Wire Color Code

By this point you have probably noticed that the wire colors on your new radio’s wire harness do not match the wire colors on your vehicle’s wire harness.  Below is a list of the aftermarket wire colors and their function.

Wire ColorFunction
YellowConstant power. Retains memory for clock, radio presets, etc.
RedAccessory power. Turns radio on when ignition is in accessory and run position.
BlackGround wire.
OrangeIllumination/Dimmer wire. Changes radio display to night mode when headlight switch is turned on.
BluePower antenna wire.
Blue/WhiteRemote amplifier turn on.
WhiteFront left positive speaker wire.
White/BlackFront left negative speaker wire.
GreyFront right positive speaker wire.
Grey/BlackFront right negative speaker wire.
GreenRear left positive speaker wire.
Green/BlackRear left negative speaker wire.
PurpleRear right positive speaker wire.
Purple/BlackRear right negative speaker wire.

Something to go by for Factory Wiring

All vehicle manufacturers use a different color code for their wiring so there is no universal color code I can give you for every vehicle.  There are, however, resources available to get these color codes.  I like to use the12volt.com to get factory wiring information.

Now, I know it would be tempting to just start matching up wire colors and get the harness connected but it is always a good idea to verify all the wires before making your connections.  I’ve seen resources give the wrong information before so it’s just a good habit to get into.

 

Testing the Wires

I’ll go through each wire one by one and describe the testing procedure you will need to follow in order to verify each one.

Power, Ground, and Illumination Wires

Power

To test these wires you will need your digital multimeter set to Volts DC (VDC).

Constant Power

This wire will be at 12V at all times so you will test it with the ignition in the off position.  You will connect this wire to the yellow wire on the stereo harness.

  1. Attach the black meter probe to ground.  This can be a known ground wire, chassis metal, or any bolt attached to chassis metal.
  2. Touch the red meter probe to the wire you are testing.  If you have the correct wire, your meter should read somewhere between 11.5V and 12.6V.

Accessory Power

The accessory power wire is at 0V with the ignition in the off position and switches to 12V in the accessory position.  This wire will connect to the red wire on the stereo harness.

  1. Attach the black meter probe to ground.
  2. Touch the red meter probe to the wire you are testing with the ignition in the accessory position.  Your meter should read between 11.5V and 12.6V.
  3. While still touching the red meter probe to the wire, turn the ignition to the off position.  If you have the correct wire, your meter should show the voltage go to zero.

Ground

Since we have already identified the constant power wire, we will use it to verify the ground wire.  The ground wire connects to the black wire on the stereo harness.

  1. Attach the red meter probe to the constant power wire.
  2. Touch the black meter probe to the wire you are testing for ground.  If you have the correct wire, your meter will give you the same reading you got while testing the constant power wire.

Illumination

Illumination

This wire will rest at 0V when your headlight switch is in the off position and go to 12V with the headlight switch in the parking lights or on position.  The illumination wire will connect to the orange wire on your radio harness, however, not all radios have this wire.

  1. Attach the black meter probe to ground.
  2. Touch the red meter probe to the wire you are testing.  With the headlight switch in the off position, your meter should read 0V.  While still touching the red meter probe to the wire, turn the headlight switch on.  If you have the correct wire, you meter readout should jump to between 11.5V and 12.6V.

Speaker Wires

Speaker-Wires

To test these wires you will need your digital multimeter set to resistance or Ohms.  You will also need your speaker tester or 9V battery.

To test the polarity of the speakers you will need to be able to visually see the speaker cone of each speaker.

  1. Each speaker will have a positive and a negative wire.  Before I start testing for polarity, I like to verify that the wires I’m testing are, in fact, speaker wires.  To do this you will use your digital multimeter to test the resistance between the two wires.  Attach the red meter probe to one wire and the black probe to the other.  Your meter should read somewhere between 2 and 8 Ohms, depending on the impedance of your speakers.
  2. BatteryNow you need to determine which speaker these wires are going to.  Grab your 9V battery and touch one wire to the positive battery terminal and the other to the negative battery terminal.  At this point it doesn’t matter which wire goes to what terminal.  When both wires touch the battery terminals, you will hear a pop from one of the speakers.  This is the speaker that is connected to these wires.
  3. Now that you’ve identified which channel you will connect these wires to, you need to determine which wire is positive and which one is negative.  For this test you will need to be able to see which direction the cone moves when you touch the wires to the battery.  The speaker cone will move out first, then back in if the positive wire is on the positive battery terminal and the negative wire is on the negative battery terminal.  It will move in the opposite direction if the wires are reversed.
  4. Now just repeat these steps for the remaining speakers and make the connections as you go remembering that on your radio harness; white is front left, grey is front right, green is back left, and purple is back right.

If your speakers do not pop or move in any way when you touch the wires to the 9V battery, you may have a factory amplified system.  In order to get any speaker response from the wires in the dash, you will need to identify and apply power to the amplifier turn on wire which I’ll address in the next section.

Amp Turn On and Power Antenna Wires

These wires are a little more difficult to test since they don’t get a voltage signal from the vehicle that you can measure.  Instead, they get a voltage signal from the head unit to turn on either the factory amplifier or power antenna.

Not all vehicles have a power antenna or a factory amplifier or both.  If you know that your vehicle doesn’t have either of these two components, you can skip the steps that don’t apply.

If you are unsure if your vehicle has a factory amplified system, one good indicator is if it is marked anywhere with a premium audio badge, then it will likely have a factory amplifier.  Some examples of factory premium audio systems are BOSE, Harman Kardon, Infinity, and Rockford Fosgate.

Factory Amp Turn On

BOSE

The factory amp turn on wire does just that, it turns on the factory amplifier.  It will connect to the blue with white stripe wire on your radio harness.  If your vehicle does have a factory amplifier, then you probably noticed that nothing happened when you tried to follow the steps above for identifying the speaker wires.  This is because the speaker wires in your dash do not go directly to the speakers, instead they are the input signal wires for the factory amplifier.  In order to get any signal from these wires to translate to the speakers, the factory amplifier needs to be powered on.

In order to power on the factory amplifier, you will need to apply the correct voltage to the amp turn on wire.  But before you start touching that wire to a voltage source, it’s a good idea to just verify that the wire you are testing is not a ground wire of any kind.  To do this, just follow that same steps given above for testing the radio ground wire.  The meter should read 0V if it is not a ground wire.

Now that you know it’s not a ground wire you can apply the correct voltage to turn on the amp.  Most vehicles use a 12V signal for the amp turn on so you could just touch the wire to the constant 12V wire.  Not all vehicles use 12V for the amp turn on though, so you should do some research to determine the voltage that your vehicle uses.  Many Ford vehicles, for instance, use a 6V signal for the amp turn on.  In a case like this, you will need to use a voltage divider between your aftermarket head unit’s amp turn on output and the vehicle’s input wire.  Since this is a special case, I won’t go into detail in this article, but I’d be happy to answer any questions in the comments or via email (scott@mobileinstallguide.com).

Once the factory amp is powered on, you should be able to return to the steps for testing speaker wires.

**Important Note**

Please keep in mind that not all factory amplified systems are created equal.  You may run into trouble identifying amplifier input signal wires on vehicles that have less common configurations such as common ground or digital signal.  I’d be happy to help you figure out how to handle these situations.  Just leave a question in the comments or email me at scott@mobileinstallguide.com.

 

Power Antenna

Power-Antenna

Depending on the type of antenna your vehicle has, the power antenna wire is used to either turn on an amplified antenna or raise a motorized antenna.  This wire will connect to the blue wire on radio harness.  Like many of the amplifier turn on wires, the power antenna wire will take a 12V signal.

Before you apply any voltage to this wire, you’ll want to ensure that the wire you are testing is not a ground wire of any kind.  To do this, simply follow the steps I gave above for testing the radio ground wire.  Your meter should read 0V if it is not a ground wire.

If your vehicle has a motorized mast antenna, then testing this wire will be a piece of cake.  When you apply 12V to the wire, the antenna will go up.  When you remove voltage from the wire, the antenna will go down.

Amplified antennas, however, are a bit more tricky.  The best way I’ve found to verify this wire is to have all your power and speaker wire connections made so you can plug the stereo in and turn it on.  With the power antenna wire disconnected (but the actual antenna cable plugged into the radio) cycle through the FM frequencies and make a mental note of some of the frequencies that actually come in.  Now apply 12V to the wire you are testing (you can just temporarily connect it to the blue wire off your stereo) and run the test again.  If you have the correct wire you should have a lot more radio stations come in clear.

 

Connecting it All Up

Now that you’ve identified all the wires you can start making your connections.  You could just connect your radio harness directly to the vehicle wires, but I would strongly suggest using a reverse harness.  A reverse harness is basically a harness with a plug that is an exact replica of the plug that was in the vehicle originally.  Then you can purchase the forward harness that plugs into the reverse harness.

Doing it this way will require a couple extra parts but it has its advantages.  If you ever want to (or need to) replace the radio in the car again, you won’t have to keep cutting the factory wires farther and farther back to put a new harness on.  Do this enough times and the factory wires will be so short, you’ll have a hard time working with them anymore.

Another advantage is that if you ever want to sell the vehicle but keep the radio you put in, you now have a plug that you can use to install the factory radio back in.

 

Clean Up

With all the wire connections made, it’s now time to clean it up to protect the wires and avoid vibration noise.  Wrap Tesa tape around the wires and plugs to keep them from rattling against metal or plastic surfaces behind the dash.  I like to secure the tape at the ends with zip ties for good measure, but this is optional.

 

Wrap Up

Hopefully after reading this, you are now ready to take on the job of installing your new radio even though the factory harness has been cut.  If there is anything you think I should add to this post or if you would like to ask me any specific questions about your vehicle, leave me a message in the comment section below.