power should come from the light switch, and the rheostat control on the front of the switch may be defective [would likely affect the interior light too] - trace the wire from the dash lights back to the switch and check for power there, including when you turn the knob to dim the lights .... the high beam indicator should connect to the dimmer switch high beam wire side someplace - a shop manual wiring diagram for your year helps

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics