65 Valiant now gauges go full when ignition key turned or running

Wow, what a saga. BUT PROGRESS. So the RTE is shot it sounds like, and you gauges are good. Good unit IMHO but they can fail/be blown. And the dash... goo to find that.

If the headlights do not work now, check for 12v at the big lug on the starter relay and then at the other end of the fusible link where it goes into the bulkhead connector. If you have a 65, then the other end of the link should screw connect to a solid terminal at the bulkhead and not go through a spade lug. Fusible link wire can get brittle under the insulation and blow, and you can't see it. You might find a 'floppy' spot in the wire or some distorted insulation to show the spot. If you have 12v after the link and to the lug on the bulkhead, then check on the inside. From there it goes to a welded splice buried in the harness, and then splits to the headlight switch, the ignition switch, and the fuse block IIRC.

Oh I just reread your post: no 12v at the big lug on the starter relay? Check and CLEAN both battery terminals, then the cable to the starter and then one to the relay, and then check the - led from battery to block, clean the connections on the block, and then the ground wire from the back of the head to the firewall; clean both of those connections.

You'll get there.