Either one of the bulbs on the drivers side is not making a good connection to the bulb socket, or the wire(s) to the flashing filiment of the bulb is open.
In the rear, the bulb has two filiments; one is the parking light, the other is the turn signal (flasher circuit).
The front uses also uses a double filiment as well. So, even though the bulb filiments are good, the posts are not making good contact in the base, or there is a broken wire on the circuit.
For craps and giggles, have you tried swapping the left side bulbs to the right side? Sometimes the filiments look like they're good, but there could be a break that even good eyes don't see.. Use an ohm meter to be sure, or swap the left with the right.
If you are sure that the bulbs are good, then remove the drivers side taillight assembly and look for a broken wire there. If not, remove the drivers side headlight assembly and look there.
Give an answer, they learn for the day; suggest a search, they learn for a lifetime.
2005 F150 STX Regular Cab (4.2 liter V6)
Last edited by Bucko; 10-20-2009 at 01:53 PM.