[COBALT]

It may be a "weak" ground or an untrue ground.

Light brightness is a function of voltage. If two bulbs are in series the bulbs will get dimmer the more bulbs you add to the circuit, but if they're in parallel they stay the same brightness with 1/2 the current, and less current the more bulbs you add.

I would look hard in how many elements are recieving power on the same hot, and whether or not they all point to the same ground. I bet they don't. They should each be on their own dedicated "true" ground to recieve the complete 12 volts. Use a volt meter to determine the voltage to that headlight. If it's not ~12V then you're not getting complete voltage to the bulb. Use the engine block or the frame as a true ground, and see if there's a voltage reading between the block or frame, and what you THINK is the ground to the headlight. I almost guarantee that you'll get a reading, which means that headlight ground isn't really a ground at all.

The inverse of this is also true. That ground may be "true", but the voltage supplied by the hot isn't 12 volts. That's why you need to check the hot lead to determine that. Shouldn't both headlights be on the same hot? If so they'll be the same brightness.

If all of this pans out, and it's still dim try switching the headlights and see if any bulb in the same position is dim. If so then you've got a head-scratcher. If not then it's the bulb.

Good luck.