look inside your engine bay...on top of the gearbox, there is a bolt thingy...coloured gold (brass) and it should have a couple of wires poking out of it.
check these wires...give them a light tug....light i said.
mine had popped out.
if you'd like a brand new reverse switch, you may borrow one i have(to test your wiring )
alternatively, if these wires do actually pull out...(they shouldnt) then touch them together to check the wiring !!
mine pulled out, very easily..the plastic holding them in, had disintegrated.
your welcome to borrow my switch....
i will try and post you a picture, tomorrow, for the location and shape, of the switch