I've had backup lights not lighting, be as simple as the bulb being loose or corroded in it's socket. I start by verifying power into the bulb socket with my test light probe. As far as the NSS: Two of the leads un-interrupt the ignition power when in the Park or Neutral position. The other Two power... and wire leading to the backup lights that is connected to power when shifter is in Reverse. Ive had three different transmissions including two manuals and an automatic in my 76, so I've spliced my own wire to the various switches I've needed, so I can't really check the color. I think the wire into the going into the backup light socket should match one of the NSS wires. Pull apart the NSS connector and jump the 12V terminal to the backup light wire terminal. I'd put my money on the problem being the bulb socket.