A 61 would have been born with 12 volts. If it still has the original generator, and you have other stuff running like a heater, it is normal for the lights to get dimmer at an idle because generators don't charge at an idle like alternators do. I seriously doubt that the problem is in your dimmer switch. However, to eliminate this, reach under there and touch the dimmer switch. If its hot, it is defective. A dimmer switch works by switching between two circuits and does not work with resistance like a heater switch.
