just went through this on my 66 ford f-100 by a simple test, you should be able to test the dimmer switch. remove the plug in at your dimmer switch, there is 3 plug ins, the center one is the hot wire, and on either side is the high, or the low beam wire, you need to jump from the center to one side, turn on your light switch, and check the lights, if they are on, turn off switch, then jump to the other side of wire harness, turn on lights and check whether or not their on, if they are, the switch is ok, it's a quick way to make sure of your next move. If one side or the other did not work with this test, it will tell you the color coded wire to follow to look for breaks etc, but as another reader stated you should also make sure the headlamps are both working by removing and testing the same way. By this simple nad quick way, I found my issue was the dimmer switch. Good luck, and hope I was of help.

From memory! pull the light switch out, behind the dash is the switch , there is a button on the bottom of it. Push the bottom in and pull the knob out of the switch. Take the nut off pull the switch down. And see if there is a burnt red wire. And check for volts there. if not the red wire coming from the starter is broken. Also check for volts at the fuse box, for the high beams. If not take the fuse box off the instument panel, some call it a fire wall. you wil fined it. Check bulbs first. Then for votage at the conectors behind the light.