The thermostat is on the heater grid in the seat. You have to take the seat apart to get at it. Put an ohms meter across the heater plug on the seat. You should get no resistance if the heater and thermostat are ok, but I can almost guarantee you have a broken wire.
It is best to take the seat out of the car to work on it (4 13mm nuts). You have to unhook the back of the seatbottom. There is a rod that goes across the back. Pull on the middle of the rod and it will pop out(putting it back in is very frustrating). You can then pull the heater out carefully. You have to lift it off the foam because it has little points that stick into the foam to stop it from shifting. Once you have the heater out you will see the thermostat, and most likely a break in the heater wire at the thermostat. Solder it back on and cover it with heat shrink tubing.
You will likely find other problems with your seat, such as the support grid falling apart and ripped foam.