it could be the amp, but we cant assume in the electronic world. testing needs 2b done. i actually run an installation shop in scar, and no, dont take your bmw to guy who works on "too many civics and such", no offence to him, but as you mentioned he mite not know the ins and outs of bmws... the shop name is safe and sound inst. 4-757-7233,
remember it mite not be the amp.... good luck,.