Something wrong with the 50 year old wiring that was designed to last 10 years tops? Seriously it could be anything from bad grounds to bad connections to broken wires bad fuses, fuse box failure light switch issues. They are all pretty simple circuits pick one and start tracing it. If the harness is original don't be shocked when you or who ever works on it make more problems just by moving stuff around.

 

Those lights are all on the same fuse !! Check the fuses !!