2001 mitsubishi galant heter dont work blows cold replaced thermostat and radaitor still cold

There is probably some kind of damper that opens and closes under the dash that allows the air from the blower to be blow across the heater core under the dash. Sometimes these are controlled electronically or by vacuum tubes. Look underneath the dash and look for small vacuum hoses that might be dangling loose or possible little wire cables that might have come off the sides.
Possibly check Haynes Manuals (www.haynes.com) to learn how to change a Heater Control Valve.