The reverse lights are wired to your NSS as stated by others. The wires are green and should be on your steering column. If they dont come on when you go into reverse you have to move the NSS left or right and try again. I just went through this not to long ago. Here is a diagram I found that helped me out. You can also look at my post to see if there's anything there that might help you as well. Good luck.
http://67-72chevytrucks.com/vboard/s...d.php?t=755946
Mike