Hi
I had this problem with mine, and found that the wire that runs to the switch on the gearbox had rubbed through the gear box mount and had corroded that badly that the copper core was green dust. All I did was replace the wire with new wire, using crimp connections, taped up all of the new part with electrical tape, refitted the corrugated cable protector and then zip tied it to various parts of the car, e.g. air con dryer, making sure that it can move freely as the engine and gear box rocks forwards and backwards.
I hope that this works for you, as I was stumped and was on the verge of replacing the switch in the gearbox, but had no need to.
Best of luck.
JamesAtthews