For the non-experienced car audio installers, if you have never installed in a BMW before, it will be difficult.
The factory wiring loom going to the factory amp has low level input and high level output. You can tap off of the high output then use a line output converter for signal. Or you can cut rca leads and solder them directly to the inputs of the factory amp. You cannot use the power.
You will need to run power wires from the battery to the new amp.
Greg Mannino
Car Audio.Video.Security.Navigation Guru
Electrical/Mechanical Specialist