If I understand correctly, we have balanced line level signals coming from the head unit. I would intercept those signals befere they enter the amp. The SVEN4 would allow the installation of RCA cables without major hacking into the wiring. It may introduce noise though. The question I can't seem to find an answer to is if the head unit outs are front and rear.

__________________
2004 BMW X5 3.0i - Mine
2011 BMW 328i xDrive - Hers
2002 BMW 325xi - waiting for a new owner