I mount my wired camera inside the 5er and have it look out the rear window. I ran the wiring over to the circuit panel and then out through the main power access door, under the frame and up to the hitch.
I was going to put a wireless sender and receiver between the truck and hitch but it is easier to just unplug the wire when I unplug the pigtail and break-away cable.
I bought the camera from Harbor Freight. It uses 4 wire telephone wire and phone jack plugs so you can add/cut/splice to the wire by using any 4 wire telephone cable and jacks. You need to keep the wires in any splice or added jack in the same order as the original wire or your video will be going to the audio. Just pay attention to the wire colors and you will be fine.