Also check your rear light connectors for signs of corrosion, the most common reason for this is a bad earth connector on the rear light clusters. As richard said check the fuses carefully to see if any have gone, if you have no power at the rear, remove the brake light switch connector, join the 2 wires together and see if they come on. If they do, u have a faulty brake switch.
If you have changed any bulbs recently, make sure they are the right type.
If you have no power on any of the lights at the rear, follow the wiring harness to the front of the vehicle, find the connecting plug and make sure it is clean and connected properly.
Hope this helps
Steve Eastman, February 2005