Did you put it in yourself? Make sure the negative and positive battery terminal's are tightned down, and look for any loose wires, if you can't find anything, take it back to the place that installed it, and tell them they messed up your car!
BTW, I wouldn't drive with the brake light on, you may have a problem with the car, unless you have ABS, then you may just have a problem with your charging system or loose terminal...