Have you checked the plug is connected? It's under the seat, if its driver side, then move the seat back, then there will be a black plastic panel, pull it off, and the wires are under there;
Yellow = Airbag connector.
Black = Seat occupant sensor.
Big plug = Heated seat wiring.
They run off the same fuse so I doubt it's that, although worth checking, alternatively could be the control unit of the seat itself? (Little box under the seat!) You can't see it unless you take the seat out and turn it upside down though!