The headlight circuit is dead nuts simple. The headlight switch has TWO power sources. The headlights ONLY are powered from a tap off the "welded splice" in the ammeter circuit, to a built in breaker in the switch. With the light switch full on, it feeds headlights power out of the switch, down to the dimmer. TWO of the terminals of the dimmer are ALWAYS hot with the switch ON. One terminal is power coming from the headlight switch, and the other is that power, selected by the hi/ low function of the dimmer, to feed out through the bulkhead and to the headlights.
The proper description for the dimmer is SPDT or Single Pole, Double Throw. This means it is switching ONE circuit (single pole) and selects which of two outputs which it feeds to (double throw)
IF THE power is getting that far, your cluster bright indicator should light when you actuate the dimmer.
The high and the low beam power, whichever is selected, flows out of the dimmer switch, through the bulkhead connector, and to the lamp harness in the engine bay.
THERE IS NORMALLY ONLY ONE ground for both headlamps, somewhere on / near the radiator support. FIND AND CHECK that ground.
You should be able to probe the two bulkhead connector terminals for the high and low beams and by actuating the dimmer switch, get first one and then the other to power up.
Next move to the lamps. Find the ground, pull out at least on lamp. Bad ground, miss wiring, or even both lamps blown. That HAS happened!!