UMDterp13 is right.
Take a test light to the terminals on the rear brake light switch located at the rear M/C.
One should have power with the key on, the other should have power when you press the pedal down.
No power with the key on? check fuses, and the wire between the switch and the fuse box.
Power at the switch on one terminal, but not on the other terminal when you press the brake pedal? Replace the switch.
Power on one terminal, and on the other terminal when you press the switch, but still no brake light? Check the wire from the brake switch to the rear light.