Had the same problem, easily fixed, here is post #952:
Headlight dimming issue resolved!!! After checking everything out with a friend who has a good knowledge of electrics it is confirmed as reported by Reporon that there is a blue wire with white stripe underneath the left side of the gas tank that connects to a dark green wire. It is the only single wire to single wire connection underneath the tank. For some reason as you rev the engine the voltage actually drops from the dark green wire. SOLUTION: Disconnect the wires, cap the green wire and connect the blue wire with white stripe to one of the black wires which are usually a constant 12V (backwards from the black ground we are used to). Now the headlight comes on with the ignition switch and the headlight gets brighter with the engine RPM's and maintains a constant brightness. Also happy to report that there has been no battery problems wiring the headlight that way.