The bulb is fine because it works fine when the headlights are off. I realize now my first message was pretty vague.
Let me expand on the subject, I have a '00 Jeep Grand Cherokee, when the headlights are turned on, pushing the brake pedal causes the left ( drivers ) side brake light/taillight to go out. When the headlights are off the brake lights work fine. It seems like it is some sort of grounding problem. I was just wondering if any of you have had this problem and wouldn't mine pointing me in the right direction. Thanks in advance.