starter relay

Agree RRR. The YELLOW wire comes from the KEY. It IS the "start" voltage that triggers the relay. By disconnecting that, and jumpering from battery power to the "push on" blade terminal on the relay......you said it cranks, right?

THAT PROVES the relay, starter, are OK

Now you must find out why you have now power on the yellow wire in crank.

Might be the bulkhead connector. Wiggle the connector try again. Remove the connector and inspect. "Work" it in/ out several times to scrub the terminals

Access the ignition switch, find the yellow and check it there With your meter connected to the yellow, twist the key to "start." Voltage?

Yes.......the ignition switch is OK, you are losing it between there and the relay. REcheck the bulkhead connector, is most likely

NO voltage at key, might be right in the connector or bad ignition switch