Check the coolant level first. If it is low, the heater core may be empty and blow cold air. The thermostat is on top of the engine; follow the upper radiator hose to locate it.
A 1999 Chevrolet Blazer with a 4.3 V6 has the heater on but blows only cold air. The owner asks if the thermostat is the cause, whether it is hard to replace, and where it is located. The expert says the thermostat is on top of the engine along the upper radiator hose and advises checking the coolant level first, because low coolant can leave the heater core empty and cause cold air.
