Thanks for the help with the wiring insight.
My problem seems mechanical instead of electrical. The back up light switch remains in the normally closed position when the transmission is in gear or not. When I removed the switch it returned to normally open.
I’ve taken the top off the...
Did you check the switch itself. Is it showing a normally closed contact?
I’m having the same problem on a 1974. I found the contact on the switch before and after installation to be normally closed.