I have a 2003 Jeep grand chrerokee lerado and i'm having a brake light issue. As long as the headlights aren't on they work fine, however when i pull out my headlights my left brake light goes out.

yes you have bad chassis ground. Look at the black battery cable, clean the end wearing some latex gloves on the acidy side, on the other side what connects to the frame/chassis needs to be cleaned down to a metal surface, here and the black 1/0 battery cable going to the engine which also should have it's electrical connection maintained...think fer sure you've got weak ground to the chassis~

clean the ground side what connects bulb socket to mounting screws too-

There should be a ground connection at the back of the car on the tail light wiring harness, for each side of the car, which explains why only one side is affected. I bet you will find that the black wire is broken somewhere between the socket and the body of the car, or the connector on the end of the wire is adrift.

the tail lamp assm. is bad. circuit board inside lens failure