Need some electrical help with my ‘68 Plymouth Valiant 100

The first thing I would do is unplug all three bulk heads connectors at the firewall. Clean them with contact cleaner , Put Dielectric grease on them and plug them back in. Also add a ground from the negative terminal to the body. The large positive wire coming from the battery going to the bulk head has a short extension near the firewall called a fusible link. They get corroded inside and at times barely make contact. If you can stretch it at all the fuse wire inside is no good. Its a start .