Not sure if this helps you out, however the rear window defroster setup with dark tinted glass window was only available as an option on the extended cab, and crew cab in the earlier 88-94 trucks. It was never made available as an option on the standard cab trucks from these years since the smaller cab and windshield defogger setup did an adequate job of clearing the rear glass as well. GM probably figured they wouldent sell many configured that way so why offer it.
I must have been one of the few interested in it though. I special ordered my truck new in 1994 and enquired about this option since i saw it in the brochure, and was told it wasnt available for a std cab. I briefly concidered it later, but now i am going to put in SS 454 driving lights. The switch for those goes where the defogger switch is mounted to the right of the steering wheel. I will be installing a smoke tint non defrost smooth rear glass.
This certainly would be cool to do to a standard cab though. Your best bet would be to pull the wiring, switch and relay from an 88-94 suburban. This way that part is plug n play since the dash and fuse box is the same or similar to your year truck. Then find a 95 up pickup and get the wiring and terminal ends that goes up to the defroster type rear glass dince those are more common. A few soldered and shrink tube splices in the kick panel area to mate the window wiring to the suburban body wiring if thats how they ran the wiring originally and Viola, rear defogger in a std cab truck.