Could be a dodgy connection on the plug or a wire that has, through movement, broken off the back of the plug or somewhere else in that circuit.
Get your hands on some contact cleaner, take the plug out and give it, and where it connects into the gearbox, a good blast. I don't recommend using anything else to clean it as contact cleaner is specially made so it dries REALLY fast so as not to leave moisture on the pins. And if you can, pull the protective cover back near the plug and see if any wires are loose or broken off. If so, re-solder them and see what happens.
Or, if you don't want to take the chance soldering it yourself, send it to an auto electrician to get it done.
Hope this helps.
Old cars, are they really worth all the problems and screw ups?.....yep!