Seen this one come up quite a few times, so here's a breakdown of how to figure out which of your OEM wires in the headunit wiring loom should go to where on the back of your new headunit. It may also serve to help people better understand how to use their multimeter to its full(er) potential.
First you'll need a multimeter (£3 from maplins last time I was in there). For most car audio applications, all you'll need is one which measures DC voltage, resistance, and preferably continuity too.
You'll also need a wiring diagram for the new headunit's loom - if you don't have a manual, you can usually find a PDF or similar on the headunit manufacturer's website, or by searching google.com
Lastly, some masking tape and a pen will be useful for marking up cables/pins once they're identified, and an AA/AAA/C sized battery will help to identify the speaker wires correctly (you'll also need to be able to see the speaker cones if you want to correctly identify the speaker positive and negative wires). Don't be tempted to use a 9V or 12V battery unless you don't care about any existing tweeters in the system. If you have rear speakers connected to the wiring loom, you may want a friend to help with step 15 below.
1) Remove the OEM (or existing aftermarket) headunit altogether. Don't forget that many OEM headunits require a code to get them working again, so make sure you have this handy, incase you decide to bail on your plans half way through (or wish to sell the headunit on).
2) Turn on the multimeter and set it to DC voltage.
3) Clamp (or touch) your multimeter's negative (black) probe/crocodile clip onto a good earth, such as a bare bit of metal on the vehicle chassis, or the outer sleeve of the cigarette lighter.
4) Ensure that the headlights are turned off, and that if your keys are in the ignition, they're set to position 0 (off)
5) Probe each of the loom/connectors using the red multimeter probe until you find a reading of around 12V. This is your permanent live wire. Label it using the pen and tape.
6) Turn on the ignition (set it to the "Acc"/"Accessory" position.. there's no need to start the engine. Probe all of the pins/wires except the one already identified as permanent live. When you find one with around 12V, label this up as the "ignition switched live"
7) Turn on your headlights, and repeat the process from step 6, ignoring already identified wires/pins. When you find a 12V signal, label this as your "illumination" wire (basically it tells the headunit to dim the display when your headlights are on).
8) Turn off your headlights and ignition. If you have a continuity test mode on the multimeter, go to step 9. If you don't have continuity testing abilities, go to step 10.
9) Switch the multimeter to continuity test mode. Remove the black (earth) probe from the good earth, and touch the probes together - you should hear a "beep", which indicates that the continuity mode is working. Now reconnect the black probe to the chassis/outer sleeve of the lighter socket. Probe the wires/pins on the loom connector until you get a beep - this is your earth.. don't forget to label it. Skip to step 11.
10) You don't have continuity testing abilities, so switch the multimeter to the resistance (ohms) mode. Remove the black (earth) probe from the good earth, and touch the probes together. You should have a reading of somewhere near 0 ohms. Now reconnect the black probe to the chassis/outer sleeve of the lighter socket. Probe the wires/pins on the loom connector until you get a reading of somewhere near 0 ohms - this is your earth.. don't forget to label it.
So, you've now identified everything you need in order to get your headunit to switch on, and operate correctly. On to the speaker wires (if necessary for your install). You'll need working speakers connected to the original loom in order for the remaining steps to work (if they're not connected, you'll need to add some temporarily, or create your own wiring scheme for them). What you're going to do is find matching pairs (positive and negative wires) going to speakers, by measuring the resistance of the speaker. The figrure you're looking for will likely be somewhere between 2 and 8 ohms, but most likely very close to 4 ohms.
11) Switch your multimeter to resistance (ohms) mode, and remove the black probe from the chassis/lighter socket. Don't forget that if you switch to Kohms, or any other scale, you'll need to perform appropriate unit conversions in order to calculate for the value you're looking for.
12) Choose a wire/pin and connect one probe to it. Now cycle through the remaining wires until you find one which causes the multimeter to read between 2 and 8 ohms. That's the positive and negative for one speaker, although we don't yet know which is positive and which is negative, nor which speaker they go to. Tape these 2 wires together, but there's no need to label them. Tip: If you're dealing with wires rather than the pins of a connector block, tape the wires together far enough down so that you'll be able to touch either wire end to either end of the battery. If you're dealing with a connector block, take 2 bits of wire 3 inches long or so, and tape them to the battery.
13) Repeat step 12 in order to identify all of the remaining pairs of wires.
14) Turn you multimeter off.
15) Cycle through your pairs of wires, connecting one end of the battery to one wire/pin, and the other end of the battery to the other wire in the pair. When you make the second connection, listen for a pop from one of your speakers. If you can't see the speaker because it's too far from the headunit position, this is where your friend comes in. Make sure either yourself or your friend is watching the cone of the speaker that popped, then disconnect the battery from the loom. If the speaker cone moves inwards, this means that the positive terminal of the battery is connected to the positive terminal on the speaker (i.e the positive wire); if the cone moves outwards, it means that the negative terminal of the battery is connected to the positive terminal of the speaker. Label each wire as you go!
Once you've finished repeating step 15 for all of the pairs of wires, you should have identified every wire. Reward yourself for all of your hard work with a cup of tea (or just get on with it, and get the new headunit installed)
Hope this helps someone (and that it hasn't been written up in depth before)
Mark
http://www.talkaudio.co.uk/ipb/index.php/topic/131643-using-a-multimeter-to-solve-headunit-wiring-hell/
