'90 Chevy 2500. No reverse lights. Where is Backup light switch?

My reverse lights do not work. I assume that maybe it is the b/u light switch? Are there other common problems for this? It is cold and snowing outside, so I want to spend as little time laying on the ground as possible. Is it triggered by the shifting lever or is it mounted on the tranny? I have a 1990 Chevy 2500 manual 4-speed pickup truck.
My Haynes manual has absolutey nothing about this switch except to list it in one of the wiring diagrams.
Thaks for your help