Headlight problem

My 94 Trooper has a problem with the driver's side headlight. I noticed that it was not as bright as the passenger side headlight (~50% dimmer), bought 2 new bulbs, replaced the old ones and headlights were fine for a couple of days, maybe as much as a week, then the driverside headlight went dimm again. It has stayed that way since. I am guessing that this is indicating some sort of electrical problem but I have no idea what. Any ideas?