Relay - at the front of the seat underneath, there's 3 relays, 2 outboard black for upper and lower control the seat heaters and a brown relay controls the power to the seat, that's inboard. Move the seat forward and use a mirror to locate. Swap out for another with the same part number to see if it's that.

 

If checked and ok, probably the thermostat(s), part of the heater skins itself.
Use a search, has been covered here before.