first thing turn the ignition on, find the reverse switch on the gearbox and disconnect the terminal put a wire straight across to join the two leads to it, the lights should light, (if its only one lead to it connect it to a shiny bit on the engine, then they should light) if they do then its the switch in the gearbox- you need a new one of them, if they dont its most likely the fuse change that, and if that doesnt work then its the wiring.
Sammy the recovery driver
November 2008