In my case is was a chafed spot in the insulation running to the nuetral switch, where it bends to fit around the lower engine case. There is a somewhat sharp corner there and it can get nipped while reinstalling the engine.
The light is switched on the ground side of the circuit with battery power being fed to the bulb all the while the key is turned on. Any time you get a short to ground in the neutral switch wire the bulb will light up.