You may want to try this: check the under-belly of the steering
column... there's a device [with AUTO transmission--don't know about
manual trans] that engages the back-up lights; it's white in color, and
can be moved manually to check for alignment. After buying my '86
Silverado, my back-ups didn't work either... I checked bulbs, wiring,
etc.--everything was good, so I went inside the cab. Checked fuses,
wiring, etc.--everything good. Next, I found device on steering column,
and it seems OK. While checking, I accidentally moved it a couple of
"notches", (thought I'd broken the damn thing) and the lights came
on--I've had no problems since.
BTW... it's in a location that's easy to bump/kick the thing, and knock
it out of position/alignment--no doubt what happened to mine.