1970 Dart Headlight Switch
YOUR PROBLEM IS NOT the headlight switch, at least as you describe it.
The inst fuse at the end of the fuse panel, works like this:
Headlight switch has TWO power SOURCES, one is UNfused and ONLY for headlights nothing else. The headlight switch has a built in breaker for ONLY the headlights
Everything else, tail, marker, dash lamps is sourced FROM the "tail" fuse
So in order for the INST fuse to have power:
The tail fuse must be good and the circuit intact
The light switch must be in either "Park" or "head"
The dash dimmer control must be turned "to the left"
If you then have power at the INST fuse, then nothing I mentioned above is the problem.
Do ANY dash dimmer controlled lamps work? Radio? Heater bezel? Shift quadrant? If so, the trouble is right at or in the cluster