The first thing I would do is check what voltage I have coming from the headlight switch down to the dimmer. Then I would check the voltage going out the dimmer to the low and high beam lights and see if their is a large voltage drop. You could have a bad "high resistance" dimmer switch and it needs to be replaced. Next I would make up a jumper wire and connect one end of the jumper to the ground wire on the fender well going to the headlight and the other end of the jumper to the frame and see if the lights go brighter.....if so then you have a bad ground.
