Bentley has a good wiring section. An online resource is this: WDS BMW Wiring Diagram System - Model Selection Needs Java to run.
That online copy is incomplete. Hotlinks don't work and locator images are missing. Better to buy it on DVD from eBay (cheap pirated copies, search for "BMW WDS") and install it to run locally on your computer. It works very well (and it should -- it was the interactive software that dealerships used). You can follow the path of a wire to different components or open a pinout diagram just by clicking on hotlinks in the diagram. The diagrams on Bentley are merely hardcopy printouts from the WDS. It is painful to use without the search, zoom, and hotlink functions provided in the software.