As Duncan says the wiring is very suspect on the sensors. If I remember it correctly they are suimply soldered together and taped over?!?
I had a problem with mine when I 1st got it, as Duncan says, having the ignition on, and engine off, and the car in reverse you can quite clearly hear a clicking from each sensor if you put your ear to each.
Member No. 143
Mods so far: LED sidelights, interior lights and number plate lamps, Rover Xenon Headlights, Kenlowe Fan, Synergy 2 and EGR Bypass