A few pics of the rear sensor install on my e92 335. I drilled the licence plate light housings (1/4") to pass the wires through so I would not have to drill holes in the body. The light housings are $7 a piece if I ever want to go back to stock.
Last pic shows the CPU box velcro'd on the ledge in front of the fuse panel. For the rest of the rear wiring, I just ran along inside the trunk panel following the stock wiring harness. It goes into the narrow panel next to the seat and window. That seat piece is easy to pop out. Once you route it down from there you can just run the wire along the door trim panel and up into the area behind the glove box.