Use a test light and check for power at the dimmer switch with the headlights on. If you have power, replace the dimmer switch that you said doesn't work and then check for power at the headlamps. If you don't have power at the dimmer, check for power on the 'red' wire attaching to the headlamp switch. If you have power there, replace the headlamp switch. If not, trace the red wire back to and through the bulkhead connector.
And, clean the forward lamp grounds to the radiator support and the instrument cluster ground.