Let’s start with the easiest situation, and work up from there. Feel free to skip ahead to whichever solution actually applies to your situation.

Wiring a Car Stereo With No Factory Harness

This is actually a pretty common situation. Even though head unit harness adapters exist that allow for plug-and-play head unit installation, it’s relatively common for installers to simply cut out the factory harness and solder in the head unit harness during install. If that head unit is removed at a later date, you’re left with bare wires. Or if you just want to upgrade to a new aftermarket head unit, you’re forced to cut the aftermarket harness out and start from scratch.

While it may seem daunting to look into your dash and see a wild tangle of wires, this is a pretty easy problem to deal with. The easiest solution is to obtain a wiring diagram for your particular make and model of car, or go online and search for a diagram or table that shows which wires do what.

If you can find out the colors of the speaker, power, ground, memory keep alive, and any other wires, then all you have to do is locate them in the dash and attach them to the corresponding wires on your head unit.

If you can’t find that information online, or you would rather just do things the old fashioned way, figuring out which wires do what is a very straightforward process. With a few basic tools like a test light, multimeter, and maybe a 1.5V battery, you can get everything sorted out in just a few minutes.

For more information on how to correctly identify the mess of car stereo wires in your dash, check out our basic car stereo wiring primer.

Wiring a Car Stereo With No Head Unit Harness

This is a more complicated issue that may actually require some fabrication. If you can track down a new, or even used, harness, that’s always going to be the easiest solution. Barring the availability of a new harness, you may be able to find a used one from a local wrecking yard or used parts outlet.

If you aren't able to locate a replacement harness for your car stereo, then you're going to have your work cut out for you.

The first thing you’ll need to do is obtain a pinout diagram for your head unit. The best way to find this is to get the model number of the head unit off the label and then run an Internet search. Even if the manufacturer doesn’t provide adequate documentation, you may be able to find pinout information on a forum or elsewhere.

If you can’t find pinout data for the head unit, that’s pretty much a deal breaker.

Fabricating a New Head Unit Wiring Harness

If you can find the pinout data, then you’ll need to use it to fabricate a new harness. The best way to do this is to obtain something called a “rectangular connector” sized to fit your head unit.

In most cases, what you’ll need is a two-row rectangular header connector with a female socket that is a “through hole” mount type. This type of connector is designed to be installed on a circuit board, but it will also work in a pinch as the foundation of a replacement car stereo harness.

You may be unable to find a connector with the correct pin spacing and the correct number of pins. While the pin spacing is important, the number of pins isn’t. You can either use multiple smaller connectors or cut down a large one to fit, whichever works best.

Once you have found a pinout diagram and obtained a rectangular connector, all you have to do is solder wires to each of the pins on the connector and then put heat shrink on each wire to prevent shorts.

If your car still has its factory harness, then there are two ways you can complete the installation. Either obtain an adapter that’s designed to plug into the harness — or fabricate one the same way you did with the one for your head unit.

You can also just cut the wires and connect them directly to your new harness, although that will just shift new problems onto the next person who tries to upgrade the stereo.

Wiring a Car Stereo With No Harnesses at All

If your head unit doesn’t have a harness, and someone cut the harness out of your car as well, then you’ll have to do a combination of the above methods.

The first step is still to obtain a pinout for your head unit and fabricate a new harness for it. After that, you’ll have to identify all of the wires in the dash to determine which ones are for the speakers, power, ground, and so on.

Ford Focus Wiring Diagram

Since there is no factory harness in the picture, you have to options to consider. You can either fashion a new harness for the factory wires that will plug into your head unit harness, or solder your head unit harness directly to the factory wires.