If your car is equipped with any of the following, then you have the harness in place for the rear seat heating (you still need the relays, the switches, the heating pads or the seats with pads installed)...

SA 522
SA 220
SA 536
SA 533
SA 496 (heated rear seats, you don't have it)
Sa 461
SA 265
Sa 261
SA 248

at least this is what the BMW manuals claim...