I don't think they all have separate amps. My 2005 3.0d E53 is only 6 speaker, and doesn't have a separate amplifier.

It has a radio module, which is in the rear wheel well near the car battery. It doesnt have a separate amplifier.

It is only 6 speaker. 2 mid-ranges in dash, 2 midbass in front doors, 2 midbass in rear doors. Sounds pretty poor actually. I burnt a test CD, then measured the sound coming out of the speakers. The 2 midranges in dash are also used as tweeters, and top out at around 8khz (which is terrible).

I haven't been able to find line level outputs to wire into yet.I suspect that they might not be there. I was actually hoping to find someone who could lend me/donate a dead/unwanted radio module so that I could track down where we can find line level circuit traces, and patch in our own connections. Using speaker level outputs then bringing them back down to line level to feed into another aftermarket amp is not the way I think it should be done.
My Radio part # is 65.12_6 972 662 IA:01..

HTH. Cheers, Scott.