2004 Pontiac Montana HEATER IN MY VAN

First check the coolant level or have it checked, most oil change type stations will check free. Usually a heater core failure will result in coolant smell in cabin or wet floorboard, also a milky film on the inside of the windsheild can be a symptom. Thermastat does regulate engine coolant temp so it does have something to do with the heater, but usually you will have high engine temps if thermastat is not working. Have coolant checked first.