Usually, headlights have three wires (for the high/low beam bulbs). The wire colors are brown, green, and black. The brown wire is the low beam, the green is the high beam and the black is the common, or ground. The black wire typically has a ground lug somewhere near the headlight. Some models run the black wire between the headlights and also have the ground lug at each light.

A dim bulb (car type, that is) and brighter bulb combination can be caused by one of four faults:
1. mis matched bulbs or bulb vendors
2. worn out bulb; a headlight has some finite life, usually less than 1000 hours
3. a bad ground on one side. If the bright side ground is good and the dim side ground is bad, the resistance of the ground is unequal and one bulb gets more voltage (brighter)
4. unequal resistance on the hot side wiring. Most headlight relays have both headlights connected to the same terminal. Sometimes, one wire is partially broken or corroded or has a crimp splice in it creating a high resistance situation that lowers the voltage