Help: reversing lights not working!?

My circuit tester says the gearbox switch is good!!

I did disconnect the connector to the gearbox switch and shorted it, but still no reversing lights? My logic tells me that shorting the wires is what the gearbox switch does, so as the lights didn't come on there must be a problem in the wiring between the engine bay and the lights at the back??

Any ideas??

Might have to bite the bullet and get a garage / auto electrician to look at it.

trace wires running spare set if need be

Finally got the reversing lights working!! Turns out it was nothing technical/complicated - a rock or something had been flicked up and hit the wiring under the inlet manifold (the good news I suppose is that it only cut the reversing wires and not some of the engine wires). I was trying to trace and re-wire the circuit when the engine end connector came off in my hands - just in the hardest to reach place (unless you are a stone of course). I've run a new power to the connector on the gear box, then run a wire in to the car and spliced it in to the wires that go to the lights. Bingo........

Thank you for all the advice.