Headlight dimmer switch

[cgster]

My headlights only work when high beam is on (68 c10 truck), so I replaced dimmer switch hoping that might fix the problem. Did not fix the problem. Truck has been a work in progress for over 2 years and has the same headlights as when I bought it 2 1/2 years ago so not sure if they even worked at that time. If high-beams work, would that alleviate a wiring issue? Could both headlights possibly be bad? I know very little about wiring, so not sure how to rule out wiring issue. Thanks