The 12 volt Cargo heater on Mandalay's is the same one Monaco used. There is a snap disc Tstat mounted on the inside area of the water bay panel. When you turn the switch ON, that activates power to that tstst. When the temp of the compartment drops below 34 degrees, the snap disc closes and the heater turns on. Follow the wiring from the heater and you will see that snap disc(1 inch in diameter). You can test with a PC spray air can held upside down. Monaco systems had a red LED at the inside panel by the ON/OFF switch that came ON when the snap disc was closed and the heater was running. Doug