I did the PDC retrofit in my 2003 X5, I had the prewiring up front, but had to add the rear wiring harness, the rear gong (by the rear right side fuse box), the controller (in the battery compartment beside the trailer hitch module) Switch module (below the stereo )and all 8 of the sensors and sensor brackets. There are different color sensors/plug types for pre and post facelift trucks. RealOEM.com was a great source of part numbers.
I spent a great deal of time in various local wreckers looking around for parts. finally I ended up purchasing the wiring harness and sensor clips from an online supplier.
The toughest part is drilling the holes in the bumpers. even though the marks are on the insides, its still un-nerving to pole holes in the bumper, my advice, go slow, measure many times and test fit the sensors!! in the end it was a fun addition, and useful too! maybe even more so than the compass mirror retrofit! lol!
good luck! lots of good write ups about PDC in here! the other good advice i got was to put down a large blanket when taking off the rear bumper cover, this will protect the plastic cover from scratches!! (and it did!)
I added the trailer hitch mod at the same time as i did the PDC since i had the bumper off already!
