This guy at Fanatics, highly respected individual who has done a boatload of custom projects over the years, has a hand-made injection molded wiring harness that allows you to bypass the factory amp and wire in your own without fussing around.

http://forum.e46fanatics.com/showthread.php?t=793352

When I did my project years ago, this connector wasn't (and still isn't) available to purchase from BMW, Tyco or Seimens. I bought a used amp and cannibalized it to do mine. Technic's solution is cleaner and about the same money, minus all the hard work I had to do!