Electrical Troubleshooting help please

[elonblock]

Was this something which just happened? Have you had or done any work to the car recently which could be related?

To piggy-back on Sam's comment, a wiring diagram, or a service manual will help you to pinpoint where the wiring to the rear lights is run. And which color wire(s) you want to follow and look for issues.

If my memory serves me correctly, do the dash lights run through a rheostat which is connected or internal to the light switch?

This would explain why the dash lights go out when the light switch is taken out of the circuit.

It sounds like somewhere in the wiring harness, the back lights and license lights are back-feeding and receiving electricity from another power source which is always hot.

Because the lights stay on even with the light switch removed.

With the wiring diagram you can follow the wires from the back of the car moving forward to the fuse box and see if there is something possibly touching the rear lights wiring harness and sending power to the tail and license lights.

If it is the original harness, the wiring is over 40 years old and connections get loose, insulation can crack and rub though leaving bare wires a place to touch and and create problems.

From my experience, it's easier to replace either the one section of the harness and be done with it, versus splicing and repairing.

Elon