Agree it is probably somewhere in the ignition. If you are pretty sure gauges "failed" or were experiencing problems, you might need to go back further than the kill switch.
My 91' has a wire that comes from the engine wire harness to the switch pannel. I had a problem with it coming loose in rough water and killing the motor. (not good in 30mph winds and big waves)
Needed to tighten the spade connection.
My point is, you may chase this for a while, keep tools and electrical connectors on the boat if you dont usually..