This is where Technic's harness / advice helps. Has he gotten back to you?
You want to tap into the wires that go to the under-seat subs. As I mentioned I can't easily get back in there to look at the wires now that the sub is sitting there (and I have the HK system anyway). The most fool proof way to identify the wire is to look at the connector that goes into the factory amp (from what I understand the 20 pin speaker connector is the same on hifi / hk)
http://f30.bimmerpost.com/forums/sho...d.php?t=873325
There's a diagram of the connector there. The sub wires are thicker than the others so they're easy to spot.