Sounds like you need a new Mulit-Function Switch.:doh: The switch is in the steering column and also controls the hazard flashers. The new unit will contain the turn signal/wiper arm & hazard flasher button. This same thing happened to my 98Cobra and it got progressively worse. Check your hazard flashers, they are probably affected also as mine were. It is a plug-n-play install.
The bulbs are simple, quick, and easy to check. Not to mention, inexpensive. Go here first.
Eventually, you'll need to replace a bulb anyway. Don't forget to replace both the front and the back. If that does not solve the problem, THEN consider the multi-function switch. However, check for a faulty ground before you look to the switch.
A multi-function switch is not a simple piece of electro-mechanical workmanship. By that I mean, it can fail and can fail over a period of time in unpredictable fashions.
Last year I had my mechanic replace the multi-function switch in my '99 Ranger. He charged me $100.00 including parts, labor, and a cute little air freshener with his garage name on it. But, that was only after I swapped out all the bulbs in question, and traced back all of the ground wires in the harness. You'd not believe all of the nonsensical, sporatic symptoms I had with the brake lights, turn signals, and hazzards until this switch was replaced.
I agree with KR...I had the same issue today on my 00 GT and troubleshot the symptom. Rear driver tail light bulb (farthest left) had one of the filaments burnt out, while the other was still working. noticed it by comparing closely viewing the filaments from the passenger taillight to the driver's taillight. I would have not have noticed it if I just glanced at it from a distance. installed a new bulb and problem fixed.
note : there's a lock washer in one of the bolt screws, pry that one out before you can pull out the whole taillight assembly.
clean the surrounding area as well as polishing the remaining bulbs so you get a brighter light when brakes are applied.
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