Have a Car Question? Ask a Mechanic Online.
What kind of car do you have?
Most of the time, this problem is caused by a brake light bulb or a bulb socket that is shorting voltage from the brake light circuit to the tail lamp circuit. when the headlamp module sees the voltage that is leaking from the stop lights, it thinks you turned the headlamps on and it opens the lights. Try removing the brake light bulbs one at a time to see if it stops it.