Remove the door sill plate / trim (will make it much easier). Pull up the carpet / floor mat from around the dimmer switch -- need to get the carpet "down from the foot board". If you have a clutch, you can use the pedal to hold the carpet back.
Not familiar with custom interiors, so there May be a little more to them
the switch is held to the floor with two screws -- 3 prong connector from the wiring harness.
Personally, I think you need to replace the dimmer switch.
I am surprised you get a "connection". I think the "insides" are an arm operating a "rotary" switch.
But, removing 30+ years of crude / fuzz around the connection will not hurt.