Have you tried touching a jumper wire from the battery terminal say on the starter to the headlight wires going into the headlight on the fender well junction block to see if that changes the brightness?
Sounds like you need to do the following. Take a voltage reading of the electrical system with a meter at the battery and record that. Then take a reading at the wire coming from the headlight switch down to the dimmer and record that. Then take a reading on the high and low beam terminals on the dimmer and record those reading. If you don't see a big drop in voltage at the headlight switch or going through the dimmer then do the same test at the headlight with terminal at the back of the headlight to see if the drop is coming from the dimmer thru the junction block on the fender well and to the headlight plug. You might also want to try connecting the ground jumper from the frame to the ground wire coming from the headlight that is attached on the fender well.
I never had any luck connecting my headlight ground on the stock fender well location. I always brought my ground wire down to the frame.
I sure hope to hear you solved the dim light issue. Sometimes those gremlins just don't want to give up the battle very easy and will fight you to the end
