Not sure exactly about the full electrics of it, but there is a heated seat switch on the dash and it is a roller switch which you can select a number 1 - 5 so that you can do the drivers and/or passenger seats. Underneath the seats there is like two white plugs, one I presume is to the seat element and the other I guess will go through the fuse box etc to the switch on the dash, you're auto sparky will know.
I had a nightmare getting and finding this heated seat switch to replace mine, I eventually got one from VW for the pricely sum of £56 and a 10 month wait !

I think I do have some spare ones kicking about somewhere. If you need one let me know and I'll see if I can find them.