Blown fuse, blown relay, blown light bulbs, short in the wiring system, bad microswitch in the brake pedal... could be any of those.
brake light switch would be most common cause
fuse, bulbs
Possible ground issue at the taillights.
BAD BRAKE LIGHT SWITCH AT TOP OF BRAKE PEDAL
The BOO (Brake On/Off) Switch may need to be replaced. If it is bad, it would cause the brake lights to NOT illuminate when the brake pedal is applied.
More than likely it is the brake light switch that is attached to the brake pedal.
Check your brake light switch at hte brake pedal.
brake switch located on the top of the brake pedal
The most likely cause is a bad brake light switch (if none of the brake lights illuminate) but if its only one of the brake lights, try changing the bulb. Even if they still work as running or turn lights because they have a double filament and may still work partially, but not as brake lights.
Check the switch adjustment on the brake pedal arm.
Check the bulbs - they are doubloe filamented with the smaller being for brake/turn signals Check the fuses
Blown fuse, bulb, or bad brake light switch.