Typical Power Inputs
A head unit usually has two or three power inputs, whether it’s a car stereo, receiver, or tuner. One is hot all the time, and it’s used for “memory keep-alive” functions such as presets and the clock. Another is hot only when the ignition key is on, which ensures the radio is off after you’ve taken the key out. A third wire, if present, powers a dimmer function for headlights and the dash light.
Check for the 12V Wire
Set your multimeter to the appropriate scale, connect the ground lead to a known good ground, and touch the other lead to each wire in the speaker wire. When you find one that shows approximately 12V, you have found the constant 12V wire, also known as the memory wire. It’s yellow in most aftermarket head units.
Check for the Dimmer and Accessory Wires
After you mark the 12V wire and set it aside, turn on the ignition switch, turn on the headlights, and turn up the dimmer switch, if equipped, all the way. If you find two more wires that show approximately 12V, turn the dimmer switch down and check again.
The wire that shows less than 12V at that point is the dimmer/illumination wire. It’s typically orange or orange with a white stripe.The wire that still shows 12V is the accessory wire, which is usually red in aftermarket wiring harnesses. If only one wire ever had power in this step, it is the accessory wire.
Check for the Ground Wire
With the power wires marked and out of the way, you can move on to checking for the ground wire. Ideally, the ground wire is grounded somewhere you can see, taking the guesswork out of the equation. Ground wires are also black more often than not but don’t take that for granted. If you can’t locate the ground wire visually, use an ohmmeter. Just connect it to a known good ground, and then check each wire in the car stereo harness for continuity. The one that shows continuity is the ground. You can also check for the ground wire with a test light, although using an ohmmeter is preferred.
Identify Speaker Wires
Figuring out the speaker wires can be a little more complicated. If the remaining wires are in pairs, one a solid color and the other the same color with a line, then each pair typically goes to the same speaker. You can test this by connecting one wire in the pair to one end of your AA battery and the other end to the other terminal. If a sound comes from one of the speakers, you have identified where those wires go, and you can repeat the process for the other three pairs. In most cases, the solid wire is positive, but that isn’t always the case. To be certain, look at the speaker as you trigger it. If the cone appears to move inward, you have the polarity reversed. If the wires aren’t in matched sets, choose one, connect it to one terminal of your AA battery, and touch each of the remaining wires to the positive terminal in turn. This is a longer process, but it works just the same.
