Ford: Heater Problems, thermastat, t kit

Question
My dad has a Ford Ranger Extended Cab 1994 / 4 cylinder.  The heater fan works fine, but no heat comes out; just cold air. What do you think it is and how much do you think it will cost to fix?

Answer
Margaret when was the last time he had the thermastat and gasket replace and coolant sys flush ? if not then just go to your nearest part store and buy a therm. and gasket  and a coolant flush t- kit .you can do this your self it is very easy.good luck