Do you mean the rear lights do not come on if you press the brake, or do you mean they do not come on under the other conditions that would cause them to light (headlights/parking lights on, hazards, turn signals)? What about the reverse light do those come on? Dome light? Does the fuel gauge work? Side markers?
If it is just the brake lights not coming on when the pedal is pressed, it's probably the switch. If none of the above items work, it sounds like an issue with the rear harness. The rear harness connects to the dash harness with a multi-pin connector right behind the kick panel on the drivers side, I would first make sure it's connected and clean the connections as needed.
If some of the above items work, but others don't then it becomes a bit more difficult. The most likely cause would be a bad ground, typically each light has only 1 wire running to it (power) and the casing that the bulb sits in is grounded to the body. Corrosion is typically the issue here as it will cause a poor ground connection.