In my gf 2000 mustang the drivers side headlight is out. The passenger side works fine. I replaced the bulb and it didnt help. Both headlights come on when i hit the high beams.
Check the plug connector to the headlamp bulb. I had one that was bad. Sometimes the light woud mysteriously go out and I had to realy mess with the plug to make it come back on. After I replaced the plug, problem solved. Bad connection on pin where it plugs into bulb caused only HB to work. And if you broke the plastic tabs off, that can cause the plug to come loose easily. When you unplug, never pry up on the tabs. Thats what breaks them.