1966 C-10 Short Step 250 Powerglide — I’m reassembling my truck after a year at the paint shop (long story). The backup lights worked before the tear down and now after reassembly are not working.

I’ve looked at the shop manuals and with 1966 being the first year with backup lights, I’m not finding a lot of information. The light green wire leads to the back of the truck where there is a connector to tie the lights on both sides together. I have no power there. My questions are:

1. What fuse provides the power / or does the power come through the neutral safety switch?
2. Does the power supply wire go through the firewall immediately below the connector for all the other rear lights?
3. Is there a methodology to test the neutral safety switch / backup switch?
4. Can anyone direct me to a wiring diagram of the backup light circuit?

Any other help you can provide would be greatly appreciated.