I agree with the stevew, it is a loop in the start wire from ignition switch to the starter solenoid that runs through the NSS.
I just wired my C4 and new Painless harness. If you need to find which wires are your NSS and which are your reverse lights, just do a continuity test. The 4 wires coming from the NSS are really 2 pair. I just tested each pair while shifting the trans from park to reverse to neutral. For the NSS, you should have continuity in Park and Neutral. The reverse light pair abvoiusly has continuity only when trans is in reverse.
Hope this helps. Good luck.