Well should be easy to figure this out. I will assume you have a foot dimmer switch. I will assume you have four headlights. Two of them light and two do not. I assume you know which set of lamps are low and which are high. (inners and outers)...So... You have a connector on the dimmer switch. The switch has three contacts. The center is a 12V input. The other two are Low and High beam. Turn on the main headlight switch. Click dimmer until a set of lights come on. Using a ice pic style test light, clip the clip to a good clean bare metal place near the switch. Touch the ice pick end into the center contact on the switch with connector still plugged to it. You may have to pull connector out a tad. Test light will light. Touch the other contacts to see which one also lights the test light. Then click the dimmer once and see if the other switch contact lights the test light. If that checks out, then you know the trouble is beyond the dimmer. Like the dimmer connector itself, the wiring, the bulb connectors, the bulbs, the ground.
You can use this same method on the actual headlight sealed beam connectors, especially on the set that does not light. It would be good to know if your high beam indicator on the dash is lit.
Many variations to these tests. Like using little jumper wires to see if you can get the other bulb set to light from the switch or run a test lead from the battery to each bulb with connectors taken off. This bypasses everything just to test bulb. A million ways to trouble shoot, but you can see the idea. Which is, 1. See where 12V is and where is isn't. 2. To test bulbs with direct power. 3. Isolating the components and wires to see who is the bad guy. Isolating is bypassing existing circuits and switches and act as the switch yourself and using temporary replacement wiring. Bla Bla Bla....
