Ok we've got it isolated to a bad cannon plug at the box cluster that contains the rpm mute switch, engine de ice, emergency governor switch, fuel valve, and the left and right transfer pump switches if you have the bladders installed in the cabin. Something is sending about 7 volts via two wires to the rpm limit warning box with the tone generator. If we pull the caution lights circuit breaker, the ignition system solenoid c.b., and the fuel xfr pump c.b. the voltage progressively drops as each of the above circuit breakers is pulled one by one.
So I guess we've got about a 46 pin cannon plug to replace. Also found bad was cannon plug going into the double tach on the N2 side. Only 3 wires on that one. Intermittent continuity between the three pins so im guessing carbon tracking due to deteriorated rubber on the back shell.