1968 with no power in cab

[Captainfab]

A couple weeks ago I lost all power inside the cab of my '68. I do have power to the 12 ga red wire at the engine side of the bulkhead connector behind the fusebox. I took the connector off as well as the fuse box, checked those wire connectors, cleaned them, bent the tabs out to help make contact, reassembled but still no power inside the cab. I connected jumper cables to the battery and ran power to the horn relay connector on the inside of the firewall, and then everything inside the cab works. Is it common to have to replace the connectors inside the bulkhead connector and back of the fuse box on the 12ga red wire? Or is there something else I am overlooking? Could a bad ampmeter cause this?