I have a 2000 Jeep Grand Cherokee Laredo V6 4×4. My brake light on the left side will not work when the headlights are on. But it will work when they are off.
First thing that comes to mind would be the wrong bulb was installed. For example. a 3057 bulb is not the same as a 3157 bulb. However not to common but something that can be ruled out.
The most common issue with this model is a internally shorted tail light assembly.
If that doesn’t take care of it, you will need to get a wiring diagram and start tracing the wiring for a short or loose ground connection.