It is always best to check electrical issues by starting at the source of the problem and tracing back to power. That way you keep it simple and organized. Start with the lamps, see if they get power from the connector at the back of you truck as mentioned by everyone else. There is also a connector for the back light at the fire wall.
Important note, these beds were grounded sometimes by a grounding strap, make sure the ends are free of rust or build up that would cause it not to get a good ground. (the bed sits on rubber bushings so the strap is important to make ground )