The OP did not state what year boat. I presumed he has the MA-RA205 head, which does have the red IGN.

As to messing with the switch. Where he interrupts the circuit, is irrelevant to finding the correct wire with a volt meter. Once he knows the wire color, which was his question, he can trace it to where ever, or take it right off the switch. If he uses a circuit hats "hot" when the switch is ON, good chance its the wrong one.