Ok so you have added the switch in the hope that the loom was there,I very much doubt the car has the loom,companies who can retro fit MB heated sets make a good living from the fact that you need the entire system fitted,the cheapest way is to buy a decent system of ebay,because basically all these heated seat systems need is fused power,Command on here retro fit the MB system you could contact them to get a rough price,my gut feeling is it will cost at least £1500.