First thing you need to do is take the lens off the rear light and make sure that the housing is grounded and that the bulb sockets have power. You can do both at the same time with a test light. Just ground the light to the housing, put the screws that hold the lens on back in, take the bulbs out, and test the pins for power. If you do, you have an intermittant ground, or your bulbs are bad. If you do not, I would check the plug that feeds it all under the brake booster....the other thing that can fail is the brake light switch itself.