1991 S-10 backup light connection

I have a 1991 Chevy S-10 pickup with 5-speed manual tranny. I would like to hookup backup lights. Does anyone know where the switch would be mounted? My taillight lenses have the white section in them for this purpose and i've checked my electrical drawing, which says you use the light green wire in the harness. Not sure where that is found under the dash either.

Your '91 already has back up lamps. I'm guessing they don't work? The switch is on the transmission.