Backup lights not working

I haven't been able to find a service manual. Everything is packed away in my shop after having a major house fire in February. What I would try next is to locate a wire on the head light switch for the dash lights. Use a test light on each wire, one at a time. You'll find at least one, and maybe two that have 12 volts all the time. Ignore those. At least one will have 12 volts only when you pull the switch all the way out to the headlight position. Ignore that one too. Another one will have 12 volts as soon as you pull the switch out half way to the running lights position. It will stay at 12 volts regardless of the dimmer setting. Ignore that one.

You're looking for the wire that has some voltage on it when the switch is in the tail light position, and it varies as you turn the dimmer. If you find that, and the voltage does adjust, everything is working up to that point. That would suggest the cluster has a problem.

Read my previous reply. From last night.

You're bouncing all over the place, and nothing has been diagnosed yet except we can assume the cluster and rheostat aren't at fault. Have you found any voltage on the wire-wound resistor, (spring) on the head light switch?