here's an easy way to find out if it is a ground problem or not before you start taking the quad apart. just find out which wire is ground on the headlight and bypass the factory ground wire. if the lights stays bright then go ahead and start taking the quad apart, if it doesn't then you should look elsewhere.

just to make myself clear, what i meant by bypassing was to get yourself a wire and directly ground the lights either to the frame, motor, or better yet the battery.