I wondered the same when I put in a GT hatch.... the wiring from the dash switch to the back drivers quarter panel is there, but I think it needs a relay and timer for it to work. I have not found a parts car with one to look at it closely. But I know the switch wire is there.
From doing a conversion on a 87 4cly to 87 v8 car. I found that both cars had the same wiring, From the dash and all the way back. The only thing that was different was the motor harrness. So it sould all be there. look on the drivers side on the car towards the back. there are wires that run through the 1/4 trim panels. I think they might be back there.
Let me know what you find. All I have is a small wire from the switch to the quarter panel, and the wire for the defroster lines is much bigger, so there must be a relay involved.
Well if its there i cant find it, i guess i will put this project on the back burner for now, i would rather search for more HP than the defrost stuff!!