Thanks, Technic, I went ahead and disassembled the subconnectors from the main connector harness as you advised to examine the leads closely. Getting the subconnectors out took me a while since I originally thought I had to push in the locking tabs.
But as you all may know, the tabs don't push in. Instead I gently 'pried' open the black harness with my hands so there was enough clearance room for each tab to slip under its slot, and then I used a screwdriver on the other end to nudge them out.
Using your pin layout as a reference and map, I was able to definitively trace each wire's destination (and hence, purpose).
All the amp-to-speaker twisted-pair wires match previous posts [see member bluelake's posts and diagrams]. All the HU-to-amp straight wires correspond to the appropriate pin holes, but definitely feature NEW color and stripe patterns.
For the 2011 M3 folks (and perhaps others) with the base/standard audio, the following are what I determined just by looking, and not by using any volt meter or audible testing:
8. Gray/White = Rear Right output from HU (+)
9. Gray/Yellow = Front Right output from HU (+), I missed this one earlier
10. Black = REMOTE IN to amplifier
16. Gray/Green = Rear Right output from HU (-), conveniently stamped '16'
17. Gray/Purple = Front Right output from HU (-)
29. White/Yellow = Rear Left output from HU (-), conveniently stamped '29'
30. White/Black = Front Left output from HU (-)
37. White/Blue = Rear Left output from HU (+)
38. White/Green = Front Left output from HU (+)
P.S. Do you still have harnesses that you are selling? They are designed to receive the big black bundle but instead of plugging into the OEM amplifier, it separates out all the wires so that they can be connected to an aftermarket amplifier's RCA-type inputs, correct? Does your harness also involve the amp-to-speaker wires or are they just connected individually straight into the aftermarket amp? Thanks for all your input (pun intended).