I'm not quite following you. Can you be more specific? What is FL, BL, and the other labeling you're using?
Actually you can't see it on the circuitboard but they are spliced together in the factory wiring.
The factory head unit (which I never had, I bought the car with no h/u) had 4 line outs but only L and R speaker negatives. It was connected to an amp in the trunk that contained crossovers for the (10) speakers, 3way pair in the front and 2way pair in the rear. Each set of speakers (FL, FR, RL, RR) shares a common negative (total 4), so I'm wondering if it affects things (ie ohms & f/b fading) if I use the factory L and R speaker negatives and tie fronts and backs together or if I should connect seperate negatives for each of the 4 sets of speakers straight to my head unit. Sorry if it doesn't make sense, typical noob eh?
edit: these are the old amp connections that I am using.
edit: found out through trial and error that you have to run speaker grounds separately.
(...success! -> factory amp bypass (my way - lots of pics) - BimmerNut.com Forums )