no back up lights

well I have no back up lights, and I just changed my Neutral safty switch, got a 2nd one, and neither of them restore the lights. so this weekend I am going to start tracing the wires to find the problem. I don't have a good wiring diagram for this so I don't know when power is applied to it. is it when the key is in the start position, or is there always power, and putting it in reverse is what completes the circut? I am going to clean all the grounds also to see if that is it. any other ideas would be great