Ed brings interesting information to the thread that I hadn't considered until he posted that the wires are hot. In the case of the 1963 and your 1964, we are both correct, because there are two different types of switching being done. I was thinking of the way that GM would wire the dome lights, where there is a hot wire going to the dome lights at all times, and the other side of the dome light (ground) is carried with a white wire to the door jam switches. In many cases, where the body builder was modifying the circuitry of the automobile manufacturer, such as turning on back up lights, they would have a separate hot lead going to a door jam switch, and then there would be a wire from that switch that was tied into the factory wiring that supplied the power to the back up lighting circuit. Many times, they would also incorporate the rear loading light on the roof into this circuit, so both the back up lights and the rear loading lights would both come on.
Disconnect the battery from the cars wiring before you start retrieving the wires. Once you have the wires out of the holes, and are certain that they are not touching any metal parts of the car, hook up the battery. Then touch the end of the wire with a 12 volt electrical test light, and see if it illuminates. If it does, then the wire is considered "hot". If it doesn't, then the wire is neutral (ground).
If you have multiple wires, and don't know where they belong, you can find out by using a 12 volt battery at the back of the car. Take a wire from the negative terminal of the battery, and connect it to a bare section of metal on the car. The take a wire from the positive terminal and touch it to each of the wires, one at a time. If a light comes on, then you know that one end of the wire that you touched with the 12 volts is the beginning of the wire, and that where it terminates at the light, is the other end of the wire. If nothing illuminates, then it is safe to say that is a feeder (hot lead) from the front of the car. This wire should begin at one of the fuse blocks, and will terminate at one terminal of the switch. The wire from the light goes on the other end of the switch. I suggest that you put some electrical tape on the bared ends of the wire where they connect to the switch. This will save you lots of aggravation and blown fuses.
If you have additional questions, or need clarification on any of the above, please let me know.