As Gas says the contacts get dirty and it will cause it to work intermitinely. I just reach down at a stop and wiggle it and it will come back on.
The top is open to water and other debrie so I filled mine in with RTV.
Mach you also be able to use a ohm meter or test light on the switch, if these still work like the older ones, when you put the trans in neutral the switch completes a ground to the circuit ..Dave.