The W124 seat heater is fairly involved (relay, timer, heavy-duty wiring, etc.), and from what I've heard the seat-pads with heating elements are NLA (no longer available), so you have to do the aftermarket route from a tuner like BergWerks (http://www.bergwerks.com
You basically have to rebuild the seats and it is a PITA, plan on 4+ hrs. per seat (don't ask me how I know).
1988 360TE AMG