I like the idea of having the switch in the back, but I don't like the idea of people leaving the heaters on after they get out of the car. I don't know enough about electronics to build one, but it would be nice to have a system that 1.) shut off the seats when you turn off the ignition, and 2.) after the seats were shutoff, the system would "reset" so that once you turned the car back on the seats would still be off. If the passengers needed their seats heated then they could turn it back on, but that way there would be no way they could be left on for extended periods by accident.
Maybe it's time for me to get a basic circuits book and do a little learning.