Yes it could be, but the factory yellow wire is from the ignition switch.

The op said disconnect the yellow wire and it works normal when it shouldn't work at all.

This leads me to believe the wiring is wrong or the relay has some connection issue.

That is why I specified checking voltage at the wires and if the signal there was the correct polarity.

These old cars will burn the wiring harness and cross wires in a heartbeat.