74 duster headlight problem help!

been haveing a problem with my headlights thay only work on high beams not low i had some issues with the original harness so i replaced the engin and under dash harness with a good uncut non abused harness everything is fine but the headlights still only work on high beams so i replaced the head lights headlight switch and the dimmer switch still no luck i even took the instrument panel and replaced the circuit board checked the cannon plug and the headlight switch plug i even traced the wires for the dimmer switch and the head light switch still won't work on low beams what else can i do?