I know it's been a while since this thread was last updated, but I thought I'd add to it since this forum has given me so much back. This is on a 2000 Jeep Grand Cherokee WJ.
First of all - CHECK THE BRAKE LIGHTS BOTH WITH THE LIGHTS ON AND LIGHTS OFF.
A while back (maybe 6 months ago) I noticed a brake light out while backed into a parking spot at dinner (using the reflection on the building's window). Checked it the next day, and everything was fine. Checked it a couple days later, still good.
Fast forward to tonight. A car came up on me kinda quick, just enough for me to check my brakes in the garage (using the reflection on the garage door) right when I got in (lights still on). Sure enough the driver's brake light was out. Went inside and came back out. My wife hit the brakes so I could see which bulb was the brake light before I started unscrewing. Now it worked fine (now with the lights off). It took me a few minutes to figure out that the lights on/off trick is key to really checking things out. Looking real close the "lights off" brake light is a lot dimmer than the passenger's side (like it was the daytime brake). So now, I was sure the bulb was bad. Put a new one in that I had in my tool box. Same thing - so not the bulb. Removed the passenger light assembly and swapped driver/passenger assemblies around (same plugs so easy to do). The problem followed the driver's assembly (which is now attached to the passenger side) - so all the internal wiring is good. Switched the assemblies back around to normal, then switched all the bulbs around, and the problem stayed with the driver's assembly - so not a Jeep problem and nothing to do with bulbs. I pulled both assemblies back out, removed all the bulbs, and did basic continuity testing between the two. Everything matched up fine. So now what? Looked a little closer at the driver's brake light socket/recptical/housing/contact. I noticed that the brake light's "single contact" looked a little recessed - finally found the culprit. Looking closer, it was due to the plastic housing/ring failing. Basically the stress of the bulb/screw-in/contacts/etc has taken it's toll over the last 14 years. Imagine that! Anyway... I took a screwdriver and pried the contact/plastic up a bit. The contact finally released (either it was glued or just stuck real tight) so that the contact is now raised above the plastic a millimeter or so. So the contact now will physically mate with bulb's contact. Works fine now.
I'll have to keep an eye on it from here on out. The passenger's assembly looked like it was cracking too. The two lower bulbs (turn and reverse) looked better. These two only have contacts physically across from each other that may spread the stressing a little. The brake light has 3 contacts - 2 on one side and 1 on the other. That 1 by itself is where it is failing so maybe it is more stressed with 2 opposing it.
Hope this helps the next guy. Attached is a picture that says it all - it would have save my thousand words, right? This picture shows the recessed contact and failing plastic ring. I didn't take an "after" picture.