What would cause a 2003 Mitsubishi Galant to have no heat and blow cold air after changing the thermostat?

it is not a thermostat problem is an actuator located under the dash that lets the water pass through the heater core, all these cars got the same problem dude good luck...

Possible the thermostat installed upside down. Spring goes toward engine. Also may be air in the system. Bleed the cooling system with the heater set to hot. The heater core may also be stopped up.