I have a 2001 Jeep Grand Cherokee and for months the left tail light would not work whenever I would turn on the head lights and step on the brake pedal. I also noticed that the left tail light was not as bright as the right tail light. I checked all the fuses and replaced the left bulb with new ones and still had the same problem.
Finally, I took the fitting where the bulb is screwed into and cleaned the three brass connections where this fitting makes contact with the tail light housing. On the tail light housing there's three small silver plates where the bulb fitting makes contact. The plastic around one of them was slightly coming apart and this was causing it not to make contact with one of the brass connections. I simply got a knife and lifted the silver plate just a bit so it would make full contact with the brass connection. Now my tail light is working fine. Good luck!