Let me see if I understand so far. I typically look at simple electricity like I do water or hydraulics. Just helps me
1- You plugged in the loom to the switch and got power to the one side of the back of the switch.
2- You checked with the plunger fully depressed and not mounted to the shifter and got no power coming out of the switch. Guess your using a test light to check both of these steps.
If #1 & #2 are correct then the switch is bad. Electrical stuff is so fund figuring out on the internet at a distance
3- If your take a test light and you've got power coming out of the switch as the back then test after the loom attaches to the next loom to see if the plug is allowing the flow of electricity.
4- If it is take a test light or wire, unplug the loom and apply power to that post on the car (not the reverse light switch loom) that goes to the reverse lights and have someone look to see if they both light up.
Just the process I would go through - not saying its the only way