assuming you have only one fuse for external lights like on my 84, you probably have a short somewhere or a ground issue. there should be at least one if not two ground wires running with your headlight wires. these should be grounded to any part of the body, usually theres small screws to use on top of the grill. you also need to make sure that the cab has a good ground to the frame as well as the box having a good ground to the frame. if everything is grounded well and lights are still not working, start at the tail lights and trace the wires back to the fire wall into the fuse block. come out from the fuse block about 6 inches and run new wires all the way back to the tail lights. i had to do this on mine and im glad i did. you might also have the low beams burnt out on your headlights as these are used more often than high beams.
