Check the turn signal and emergency flasher relays. These are located behind the dash and are located near the steering column. You can usually see these with your head near the brake pedal. There are two of them - one for the turn signals and one for the emergency flashers. They are aluminum cans about 1-1/2"x2" with 3-4 contacts on the black plastic fiber end. Turn your turn signals on and feel for the "clickng" inside them. The one you feel/hear clicking is the turn signal one.... you want to change the other one. Caution, these are NOT THE SAME and have different part numbers. Check to make sure someone hasn't put the WRONG ONE in, or got them switched around.
BTW: Do the turn signals work properly?
If this doesn't fix it, check the brake light switch mounted above the front of the brake pedal arm. It may only need adjustment, but they are easy to replace too.
Sounds like the turn signal switch could be worn out. The brake light circuit runs through the switch...try "jiggling" the turn signal stalk a little while pushing the brake pedal. Have someone see if you have lights.