As said , need to know year??? If it is two separate lights (headlight and parking) then they have separate bulbs, probably separate wiring and ground. Their common connection would be the switch possibly. Or any terminal block they both share. If both have a wire from the bulb housing to the same ground, then that is probably the problem. If they have different ground spots, probably not ground problem. At any rate, play around with the ground while lights are on, try to establish another ground with a test wire. Check switch connection. Check any other common connections. A dim light does usually mean ground, you are correct.
