My brake lights works fine with the headlights turned off. When the headlights are on and I step on the brake my left brake light goes out. I’ve changed the bulb and it didn’t change anything. This is happening on my 1992 Jeep Cherokee and on my daughter’s 2000 Grand Cherokee. I wish I could talk with you guys but I don’t know a number to phone in with. I listen to you on the weekends here in Shreveport, La.
The “guys” you’d like to talk to (Click and Clack) don’t post answers here, only and occasional question. But we mere mortals are always happy to help.
This is likely a corroded lamp socket contact. Sometimes this will happen also if the lamp is not seated properly in the socket, not fully twisted. Worst cas it’ll be a frayed or broken wire.
Probably not your problem, but if you are testing the brake lights with the engine off and one of the turn signals is on, odd things can happen. The turn signals don’t flash with the ignition off, but they can cause a brake light to not work – at least on some cars. Took me about half an hour to figure that out once.
I never worried about why. I just resolved never to make that particular mistake again.
My money is on a bad ground.
My money is on replacing the bulb socket. I had a problem with my trailer and checked it 10 ways to sunday, all tests were fine but ended up replacing the socket and solving the problem.