Have you checked the globes, it's seems pretty obvious, but you would be surprised as to how many people forget.

If you have checked the globes, the next thing that I would do would be to bridge the brake light switch, which is located on a bracket that is fixed onto the brake pedal bracket, the brake lights work on a switch that activates the lights when the plunger is released.

Disconnect the plug to the brake light switch and then bridge the two terminals that run to your wiring harness with a paper clip.

Let us know how you go!