Well if you just have a standard wheel just now, there is no Seat wiring between the wheel and the head unit.
The bit most people buy is just an adaptor between the seat wiring and the new stereo.
I'd say the Seat wiring will be expensive, if its even possible to do such a thing. I'm not 100% on this but I dont think it will just be wires. There must be some kinda control unit too?