I've had mine apart so long I cant remember how the dimmer switch functions. If it worked correctly would you just twist the headlight switch to dim or brighten the dash light? I know the rheostat can fail, but you said you changed the switch. It's a little more work, but if you have changed the switch and checked the fuses the next thing to do would be to get out the multi meter and start checking for voltage. I would start at the fuse block and work my way up to the dash lights. On the opposite hand you could pull one of the bulbs from the instrument cluster and check for voltage there. If you dont have voltage then start working your way back until you find it.