If there is a relay then you can simply ground a leg of it and get power. Or you could add-a-fuse and provide power. As long as the heater itself has wires goin

If there is a relay then you can simply ground a leg of it and get power. Or you could add-a-fuse and provide power. As long as the heater itself has wires going between the seat and the ECU / relay box then you have a way. I doubt the manufacturers would go this far, but it is entirely possible for the relay / power electronics to be an integral part of e.g. the seat control module, then you'll have a hard time.