I'm just posting this bc I asked some time ago and nobody knew what the issue was. I found this so this is not my work. I thought my light didn't work when I turned on the car but it was really just when my lights were on(of course the worst time). Found this on another forum and saved a good deal of money. This may have already been posted but just wanted to make sure. Seems to be a very common issue.
Jeff | Jan 27, 2007 | Reply
First, I want to say thank you for the info, you saved me money and aggravation. Second it took mea little while to figure out what contact I was looking for. So I would like to provide a little more detail to make it easier for others in the future.
This is the corrective method for your brake lights not working when your headlights are on(2000 Jeep Cherokee Larado).
1) The cause of the problem is heat from the bulb.
2) Remove the light assembly as if you were going to change a bulb.
3) Turn the socket and remove it from the taillight assembly and set it aside(check to be sure your bulb is not burnt out).
4) Look at the opening, you will see 3 flat chrome colored contacts on the assembly.
5) Notice the one by it self at the top of the opening, that is the “ground”.
6) If you look closely you will notice the plastic around the ground contact has shrunk a little bit, this caused the contact to pull away from the socket, causing the problem.
7) With a small flat head screwdriver, VERY GENTLY pry up the contact so it stays slightly upward by itself.
8) Re-insert the bulb assembly back into the housing and press your brake petal with your lights on.
9) You should be in good to go!
10) Tools: Phillips head screwdriver and a small flat head screwdriver.
11) Time : 5 minutes per light