If everything else in the car works, including starting the car, sounds like you have power to the switch. I also assume you have the ground connection from the battery to the right inside fender.
Start by unplugging the left headight connector. On your 71 you should be able to do this from under the hood. Measure the green wire to a frame ground. Measure the tan wire to ground. Should have power to both of them with the light dimmer switch on.
If you do, it's a bad ground. If you don't, check for 12 volts on the blue wire on the headlight switch (switch on). This is the switch output wire to the lamps that goes to the dimmer switch.
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