While ground faults are a good working theory, I just checked the wiring diagrams and the PDC sensors themselves are not grounded - they are wired all the way back to the PDC module (in the trunk/boot). The PDC module itself uses a ground comb (X493) in the trunk/boot area, just in front of the spare wheel.
It would pay to check this comb (it's easy to get at) but I can't see this being the issue. You never know though...
I'm looking over the diagrams to see if there is any other connection between the AEs and the PDC...
BTW, the parking sensors don't "pick up light" - they use ultrasonic sound waves to detect near objects - kinda like a bat using echo-location.