2003 BMW Z4 Car Stereo Wiring Guide - ModifiedLife Whether your an expert BMW Z4 mobile electronics installer, BMW Z4 fanatic, or a novice BMW Z4 enthusiast with a 2003 BMW Z4, a car stereo wiring diagram can save yourself a lot of time. Automotive wiring in a 2003 BMW Z4 vehicles are becoming increasing more difficult to identify due to the installation of more advanced factory oem electronics. E85 (Z4) amp wiring diagram? - The Ultimate BMW Forum for Jul 11, 2014Hey, I'm in the process of replacing the factory amp in my 03 Z4 with an Alpine V12 in prep for a new head unit. I found the wiring diagram for the factory amp and it shows one output per speaker which makes me wonder if the amp also acts as a crossover network for the speakers. If so, I will need advice on a 3way crossover to prevent sending lows to the tweeter/highs to the front woofers.