You guys are making a great case for Musicar Certified Sound systems. I got the OP's question, but I've been taking care of actual customers today, and for the record, free tech support for customers is for scheduled installs or as we get freed up, and free tech support for non-customers is when I have time, and comes with a free sarcasm entree.
But before you blow something up:
There is a solid black lead which is actually a coaxial lead, and there is a thinner black wire. You want the thinner black wire, which mimics the turn-on of the HiFi amp and so doesn't shut off until all the doors show closed and are locked and the trunk shows closed (ditto the hood IF you have BMW security), and all that is after a delay. So basically, if you don't go out of your way, this wire will test as having 12V on it most of the time. (I assume you guys are using a voltmeter to test the wire?)
If you have ANY sense at all for wiring, you can tell them apart. If you read this and say, "But how can I tell them apart?", please close the browser window immediately and make an appointment with someone who ought to be doing wiring in a BMW M3 vehicle.