they are not brake lights they are fog lights and they turn on because it is for the car behind to see you
Possible ground issue at the taillights.
Break lights come on when you step on the break, they do not come on when the head lights are turned on.
Have you checked the fuses?
You have blown a fuse, look for an acc. fuse out.
Brake lights should NOT be effected by the headlights. Brake light function is completely independent of the headlights. Assuming you are not refering to the tail lights, or marker lights, which are supposed to come when the head lights are turned on. Not having brake lights when the head lights are on is a big safety problem and should be corrected.
Just pull the parking brake up one click. It won't engage the brake, but it bypasses the headlights.
The brake lights should not come one until you step on the brakes. The tail lights should come on when the headlights are on. If they're weak, maybe there's dirt in/on the lenses or maybe the bulb is old and clouded. Change the bulbs and see if that improves things.
Check the fuse for the headlights. It is probably open.
Fuse for parking lights probably blown.
Often this type of behavior is caused by a bad ground for the lighting circuit. Check the ground circuit for the brake lights.
bad connection/short or burnt out brake bulb. Check the harness both under the steering wheel and at the lights.
ummm... your brake lights should ALWAYS work whether your headlights are on or not, that's how people behind you know you are stopping.