Pretty sure it's on the drivers side inner fender near the ECM. I'm doubting that that is your issue though. I'm betting the wiring in the headlamp plug is going bad, or where the splice for all the grounds is located in the front lighting harness. The ground on the inner fender is for all the front lighting. All your front lights would be dim if that was where the issue is.
IMHO if it where mine I wouldn't even bother trying to fix it. I would either buy, or build a headlamp harness, and be done with it. You'll not only fix the issue you'll have brighter lamps, and relieve the headlamp switch of the full current draw of the headlamps. Not to mention being able to convert your lamps to H4 style lenses, and upgrade your bulbs to higher wattage bulbs if you chose to do so.