If its a disco 1, put a quarter between the leads on the pass. side of the rear glass defroster/defogger. Then turn on the rear defrost. It will send a current to the third brake light, and let you get it out of park. This will get you home, but remember to everyone behind you it will look like you are on the brakes all the time.
If this works, than your brake switch as mentioned above needs to be replaced. Its a 17 dollar part at the dealership, but is a bit of a pain to get at. You need to remove the cover under the driver's side dash, and look up on the brake pedal shaft. There is both a cruise control switch and a brake switch. The cruise switch has a big hose going to it, and the brake switch only has electrical wires going to it.
I did mine a few weeks ago, and worked like a charm. However, I messed up the cruise control somehow. I need to make sure the connector is secure and the hose is seated.
Good luck!