I have a OEM head unit. There are no other switches or controls other than what is pictured. I had it set up that way as you suggested too, but the door speakers sound mono, not stereo. Also, starting from the middle notch on the balance knob all the speakers come on. When I turn it all the way to the right, the door speakers turn off, but sub stays on. I turn it all the way to the left, the sub turns off, but doors speakers stay on.
I'm not sure what you mean when you said, "Make sure you wire the inputs this way from the head unit as well." I think you're referring to the red and white rca plugs coming from the radio right? I have only one plug in each color, but the amp has four inputs. I have 2 "Y" splitters for each RCA plug. I plugged one color into A+B and the other into C+D.
BTW your M3 was the first car I noticed when I came out at the Sharks VIP that one night...I thought to myself...DAMNN SLAMMED!..hehe..someone must be doing veryyy well...:wink: