Where is the reversing light switch on a 1987 Ford cargo 0813?

I'm in the middle to trying to get my reversinglights to work - all bulbs, connections and wiring checked out OK. The only thing I haven't checked is the reversing light switch which, I presume, is one of the two sets of wires that attach to the outside of the gearbox. Unfortunately, it's not immediately obvious to me which set of wires it is! Can anybody help?