Looking at the parts diagram, it appears that the cable bringing "juice" to the seat is part of the main wiring harness from the vehicle. Certainly to big of a job to change and also too expensive....
The seat module is powered by a 12V 30A circuit. For the backrest and lumbar, you have a 10A circuit to the seat. I am pretty sure that the 30A circuit is not part of your harness nor fuse box (at least, I did not have the 10A circuit to add my lumbar support).
Do you have the big yellow connector? (
https://plus.google.com/photos/11743...38929768760203)
The two larger blades are for the seat module power (12V 30A) The two smaller pins are K-Can. The little pin alone is the lumbar/backrest power circuit. The bottom connector is the one from the vehicle harness (lumbar not present). If you could find the blade connector and the associated part from the vehicle haness connector, the job could be quite neat. If not, I would use a Molex connector to bring the current to the seat.
This said, I think it is doable, but you need to bring 12V from the battery or an accessory circuit. You may want to spend some time on BMWTIS.com to explore the electrical diagrams and figure out the best location to start that circuit.