This article explains how to identify wiring using typical colors and a multimeter or a simple light test.
Car Stereo Wiring Harnesses and Interfaces Explained – What Do the Wire Colors Mean?
A head unit usually has two or three power inputs, whether it's a head unit, receiver, or tuner. One is always hot and is used for "memory keep-alive" functions like presets and the clock. Another is hot only when the ignition is on, which is what keeps the radio off when you remove the key. A third wire, if present, powers a dimmer function for the headlights and the dash light.
Set your multimeter to the correct scale, connect the ground wire to a known good ground, and touch the other wire to each wire in the speaker wire. When you find one that reads about 12V, you have found the constant 12V wire, also called the memory wire. This is yellow in most aftermarket head units.
After you have marked the 12V wire and set it aside, turn on the ignition, turn on the headlights, and move the dimmer switch, if available, all the way up. If you find two more wires that read approximately 12V, move the dimmer switch down and recheck.
