On my 2006 Holiday Rambler, the system heat switch supplies 12 volt DC power to the snap switch, but the light will not come on until the snap switch actually closes and supplies power to the heater. As soon as the wet bay temp rises above 45 degrees the snap switch opens turning off both the heater and the light on the panel. In other words after turning on system heat the light and heater only come on when the snap switch closes. If your tanks have a lot of liquid in them they will keep the wet bay warmer longer than empty tanks.