if your right tail lamp is working then it is not your fuse. if your 100% sure its not your bulbs then it sounds like either the gound is loose or an open between your headlamp switch and the left tail lamp assemblly.
I would check for voltage at your bulb first (should be a brown 14 gauge wire)
-If you have voltage then check your ground.
-If you dont have voltage, start tracing the wire harness back towards the front of the vehicle until you find a connector ( there are 3 or 4 between the headlamp switch and the actual assembly i think)
Disconnect the connector and check for voltage there.
-If you dont have voltage then reconnect the connector and keep tracing the wire until you find the next conector.
-If you do have voltage then you know the open is between that connector and the tail lamp. start carefully inspecting the wire between the two points until you see the open. Repair the open and check to see if your tail lamp works.
If you have not found an open and are all the way up to the dash, then I would replace your Headlamp Switch due to being shorted to open internally.
In the event that you have good voltage at the bulb, a good ground, and have either replaced your Headlamp Switch or verified that it is operating properly then high resistance could be the cause, but that would be last on my check list.
*NOTE*
Ford did issue a TSB in reference to excessive moisture in connections. The correction is to add new dielectric grease (F8AZ-19G208-AA) during connection repairs. It is also noted, not to use this grease for high voltage applications, such as spark plug wires, coil wires, ect.
