How hard to replace headlight switch on 67 Valiant?

There is no headlight fuse. The headlights (only) get power from the black ammeter wire splice, with a breaker in the switch

As mentioned, FIRST place to look is to check power at the dimmer switch. There are three wires. Two are always hot when the headlights are on. The third one comes "hot" when the switch is cycled, and one of the other two goes dead.