What causes one headlight to be brighter than the other?

it is possible that you have two different rated bulbs, check to see if they are the same bulbs. it may be that one of the bulbs is incorrectly fitted which would give a different beam pattern and brightness. check the earth for the two bulbs, if one is poor and gives a high resistance it will affect the brightness, or it may be that the dull bulb has come to the end of its useful life and requires replacement. hope this helps, robb Different intensity bulb or poor ground.