Wiring for reverse light switch
 

On which wiring harness does the pigtail connecting wire from transmission switch for backup light connect to?

You should see a flat connector under the dash with one light and one dark green wire. One of those wires should be hot with key on, and the other leads to your backup lights. Run two wires connecting those green wires to your transmission switch. 99% sure I have the colors right for the late 60s and early 70s cars I have worked on.

Sent from my SM-G965U using Tapatalk