If they work it I've been wondering if it would be feasable to remove the existing seat cover, fit this to the seat and then re-fit the seat cover over the top. It would then be simple to do away with the cigar lighter plug and hard wire the heaters via an in-line fuse and a discreetly mounted switch.
My concern would be how safe and durable these things are. Or there's the expensive carbon "fibber" !? option from these blokes http://www.carfi.co.uk/html/heated_seats.html