Vince, to make it sure, ask here on the E32 board.
http://www.bimmerboard.com/forums/e32/
Also mention from which built year the donated heated seats come. That might be different.
There are 2 different switches for the seat heaters, the old ones on low, on-high and off, the newer ones have under the push button a electronic control.
The wiring/plugs might be different.
On my website below there are the wiring diagrams for the E32 for each year, that will help you.
The switches for rear seat heating are located left and right of the cigarette lighter in the rear at the console between the front seats.
Wires should usually be already there under the carpet, but that also depnds on built year.