Headlight Switch Schematic

Things to check. Check CAREFULLY the condition of the terminals in the switch connector. They may be loose causing heating and damaging the terminals, and conducting heat into the switch.

Make certain the switch is actually getting power at B1. You may have an intermittent connection there. That wire joins the black ammeter wire at the "Great Big Welded Splice" and those can and do fail. Wiggle the harness while having someone watch the headlights when on

Go over and check the path to the headlights..........the dimmer switch connector and the dimmer switch, and the condition of the bulkhead connector terminals

What are you using for headlights? Are they much higher wattage than originals?

There is the very real possibility that you keep getting "modern Chineseoationized replacements which are crap. Before I tore my car down, I had bought an NAPA dimmer switch. It lasted about 20 "stomps" before falling apart. I don't drive much at night