I added heated seats to my 2LT '08 coupe a couple years ago. After a lot of research, I purchased the Heizgerat Setzt III heated seats kit form www.heatedseatkits.com
. This company supplies Mercedes, Porsche, Audi, etc. I think I paid around 200 bucks for the kit and probably another 20 bucks for the installation kit. (You need the installatin kit because it includes a 2 step drill bit needed for mounting the switches in the factroy locations along either side of the ashtray door). It is a pretty easy job but you must take the seats out and take the covers off; about a two hour job, I think. When finished, the installation looks factory. If you want to send me a PM with your e-mail address, I'll send you some pictures.
I researched the possibility of adding the factory heated seats, but it is a next to impossible task and would easily cost over $1500 to accomplish. Probably more than that.