I bought this blazer for my daughter and the heater worked when we got it. The core is not leaking it just fails to heat when the selector is moved to this position. Any ideas anyone?

Does the air flow change outlets when the different modes are selected? If so, check your coolant level. If good, the heater core will need to be flushed. Disconnect both heater hoses and spray your garden hose into one tube then the other until the water flows clear.

If you hear a gurgling sound behind the glovebox, check your coolant level in the radiator with the motor cold...it should be full, if not fill it full then replace the radiator cap.