Where is the backup light switch on a 1989 GMC pickup with automatic transmission?

On a 1989 GMC pickup with an automatic transmission, the backup light switch isn't a separate switch you'd find on the dash or elsewhere. Instead, it's integrated into the transmission itself. It's a mechanical switch activated by the shift lever's movement into reverse. There's no user-serviceable switch to replace or adjust; if your backup lights aren't working, the problem is likely within the transmission's switch or the wiring connected to it.