I am doing an HID reverse light upgrade on my car as well. Helps enormously with the backup camera, especially since I park pretty tight so I want to be able to see the camera easily at night as well.
I am wiring mine into the switch for my windshield wiper (I deleted the actual wiper), first speed setting turns both lights on, 2nd speed setting is going to make them "strobe" (flash at an interval, I will see how fast I can get it to be with HID's, before I destroy a bulb or ballast).
Tailgaters will be banished from my rear bumper!!
