There are two filaments in such a bulb. One filament lights up when you turn on your headlight and the other lights up anytime you apply the brakes, even if you don't have your headlight on, and if you have the lights on and apply the brakes, the filament that is on stays on and the "stop" filament lights up, so you have both filaments on in this case. Knowing this, you should be able to troubleshot and determine what's wrong.