One bad ground will make that headlight try to find a ground by backing up through one of the filaments of the other light. The added resistance causes a very dim glow instead of normal brightness. it also messes with the current flow when the dimmer switch is operated. That's also the reason brake lights, turn signals, and tail lights do oddball things when one bulb socket isn't grounded properly.

To test my theory, try disconnecting the headlight ground to one side on purpose and watch what happens.
Jerry


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