Trailbeast is right about the switch pinout. The middle pin wire goes up to the starter relay to provide a ground to it when the trans. is in neutral or park. When it's in any motion gear no ground to the starter relay won't let it start.
The outer wires are for the backup lights. They wire just like any other 2 wire switch does. Some early models had a switch on the shifter. Generally anything newer than 68 has no switch on the shifter. It's all done at the transmission switch.
