Voltage at Taillights 66 Dart?

You need to check under load if possible. A bad connection can sometimes check good for resistance. I realize that is difficult if it won't work. It is unlikely to be wiring, more right at the connection, AKA the crimp onto the wire, "let's say" at the dimmer switch, the actual connection from the connector to the dimmer switch, and the contacts right in the switch

Same deal in the bulkhead connector. Wire crimps, and connections right at the connector terminals. If you have not pulled the bulkead connector apart and inspected you should do that.