Just went through this on my 70 Cuda. First of all the switch actually works opposite what your thinking. When your tranny is in reverse the switch is released from being pushed in which complets the circuit, when the car is in any other gear the button on the switch is pushed in which in turn interrupts the circuit. I beat my head against the wall on this and I ended up having the wrong switch, one was shorter that the other. If you have changed the switch make sure the new one is the same as the one that worked. The difference is in the length and they have to be lying next to each other to even see it. If it is the same start at a location where you know you have power and track it to the back until you lose power on your tester. Common problems are grounds at the receptacle, contact points inside the socket and supposed new bulbs that are junk.
