I think the wiring is allready in the E30's for heated seats. I might be wrong but its worth a look. There are two wires with a plug in all three of my E30's that just sit there under the seats, and I think they are for the heated seats. I cannot verify this, I need a Bently manual to see if the wires are for the heated seats.
A lot of car manufacturers will put the full wiring harnesses into cars to save money. Example: since the 70's all ford cars that come with all the accesory wiring, like if you had a ford truck and wanted to put cruise control into it all you have to do is put the cruise control module, computer, and the buttons on the steering wheel... and there you go. Its a lot cheaper for a car maker to stamp out the full wiring then to have production runs of just the basic, "no accesories", wiring harness. I hate to say this but the evil Chevy's even do this I think.
Later.