an electrical nightmare , please help , 1972 dart

Chase the ground for the headlights where it attaches up front, to the rad support, wherever. The harness will come out of the lights, and at some point will be an eyelet and screw. "It might pay" to pull a headlight out so you can access the terminals. Suspect the bulkhead connector again for voltage drop. Check the dimmer switch. Path is headlight switch, power to dimmer, select high or low, those wires through the bulkhead, and out to the headlights.