I would guess the heater wiring may not carry the amperage to operate the seat motors, with regards to the seat heaters they draw a lot of juice so again amps may be an issue using the heater wiring. I would add, my car has cloth manual seats, apparently it was an option instead of electric leather? The heaters are there as is the electric lumbar support, I originally would have liked leather but found they are either cold in winter and burny bum in the summer, cloth fits the bill all round so I am happy with them. I added that info in case you go loom hunting, you might stumble across a vehicle with no seats and end up with wiring that does not suit electric leather, now at least you know there are alternative looms.
regards
Dave