So, E91 iDrive, upgrade, SQ oriented, stock appearance, demo worthy for sound, not show. Done plenty for clients, needed to do mine. Decided to do a fully active, DSP-tuned, front/sub system with rear L-R ambience channels with massive time delay, front high/mid/midbass/rear/rear-subwoofer system.
Intercepted HU out and ran into Zapco balanced Symbi cables.
This system runs the front and rear HU outputs into separate Zapco Symbilink cables.
The signal routing is more complicatied than I would have liked. I only have 2 pair of HU outputs. The front output goes into a Symbilink "Y" adapter. One side feeds a DC650, the other feeds a SLB-U, which then feeds a DC500. This gives me a knob up front to control the sub level.
The rear output feeds a "Y" adapter which then feeds two cables, one standard, one modified. These are run to a DC200 (50x2). The DC200 is running rear speakers, and I can select which of the two cables to plug in (standard stereo or L-R).