I have a Jeep Grand Cherokee 2000 that had the same problem. I did not notice it until a kind soul shouted at an intersection that my brake lights were off! In New York City, it is illegal to have non-functional head-beams and brake lights. Luckily, searching Google, I found a fix that worked for me. Here is a copy of the procedure:
1. Remove the tail light assembly. Most of you know how to remove the light assembly if you have changed bulbs, so I will not cover that.
2. Disconnect the wiring harness by sliding the red locking clip to the side in order to press the side of the male connector and pull apart.
3. Once you have got the tail assembly away from the vehicle and on a bench, remove the light socket for the tail/brake light. You will see three silver contact points inside the black plastic ring. Two are grouped together and there is one that is opposite of the two. The one by itself is the ground; the other two are the contacts for the tail light and brake light. The problem lies with the ground connector. (By the way, I tested continuity with all lights and contacts including the turn signal and the reverse light to verify that the circuit board itself was good, and it was).
4. Clean all contacts with a Q-Tip and some sort of contact cleaner - rubbing alcohol will work.
5. You should notice that the ground contact is depressed into the plastic and the plastic itself is slightly warped down. I was able to use a small pair of needle nose pliers that have a forty five degree angle to press up on the contact from underneath using the opposite lip of the hole as leverage (very carefully). Once it had moved up a bit, I was able to wedge a mini-flat head screwdriver in between the metal contact and the plastic to force the contact up even further (again, very carefully). Once it looked even with the rest, gently reinsert the connector and test.
2000 Jeep Grand Cherokee 4.0L
Wednesday, March 1st, 2006 AT 10:18 PM