2003 BMW 525 OEM amp bypass

I'm doing an install for a buddy of mine and I've been racking my brain (crammed in his trunk) trying to figure out how to make it work. 

I installed a two channel for a sub just a few monthes ago, supplying it with a signal from what I understood to be a low-level amplified signal from the head.  My previous research found that the signal in a newer BMW is amplified first at the head and then again at the OEM amp.  I was curious when I completed the install as to why a "low-level" signal from the head peaked the new amp with very little on the gain input.  Comfortable listening volume for me is just above ZERO when balance with the front stage speakers.  Anyway, to continue...  Now I'm attempting to put in an aftermarket 4-channel and bypass the OEM amp.  I'd really like to just cut the OEM amp out of the mix, for obvious reasons.  I know what I'd like to do will work, but I need some help figuring it out. 

There are two harnesses connected to the OEM amp:  one 11-conductor (+12V, Gd, remote, and 4 speaker level pairs) and one 20-conductor (assumedly 10 speaker level pairs).  I just assumed before beginning that the 11-conductor carried the 4 channel input to the OEM amp, but I had my meter, a dummy speaker, and invented some creative ways to find out that I was wrong, the 4 pairs do not seem to be inputs.  Maybe I'm just an idiot and doing something wrong.  Does anyone out there know for sure?  ...or have an accurate wiring diagram for the 10 speaker set-up at the OEM amp?    responses greatly appreciated!  thannk

Brad