Your brake light switch, or stop switch, is a possible culprit. This is the switch on the brake pedal that switches on when you step on the brakes. On my Plymouth Acclaim the spring loaded plunger is pushed in normally. Stepping on the brake releases the plunger. The switch makes electrical contact when the plunger is out. So if this switch ever breaks off of it's mounting your brake lights would stay on. It also has a tendancy when off its mounting to wrap itself around your steering column, be careful.