check the dim-dip resistor bolted to the front pannel near the driver side headlight. its about the size & shape of a packet of polos, with a yellow wire coming out of each end. Follow the wires up the chassis leg till you find a 2 pin plug and disconnect it. They can fail or the wires rub through and short to ground!
The other thing to check is you have the right headlight switch, make sure it has the same number of pins as the plug!