Assuming stock headlight wiring which has a 3 position terminal board on the driver's side inner fender near the front ...
Attach a long wire to battery + with the other end stripped back just 1/4 inch or so. Touch one terminal at a time and see what happens on the front of the truck. One is high beam, one is low beam and one is for the parking light.
Because your high beams work, you have proven the circuit is good from the battery, through the headlight switch and down to the common terminal on the dimmer switch.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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