The switch is behind the shifter ON the transmission. The wires come up about the center of the firewall (engine side) and end at a 4 pin plug. 2 wires for the reverse lights connection and 2 for the neutral safety switch. The 2 backup wires will show connection with the shifter in reverse. The neutral safety switch will show a connection in Park or Neutral. This is assuming you have a continuity tester. If not, you can rig up something with a flashlight bulb, batteries, wire and some tape. Let me know if you need more detailed help.
I had the same problem. I took a wire and jumped the leads on the neutral safety switch plug where it connected at the firewall. When I hit the right combination, the backup lights illuminated. I replaced the NSS and haven't had any problems since.