heat problem

Hello I was wondering I have a 97 dodge dekota and when i turn the heater on i have warm air comming out as soon as i turn the fan on i have cold air does anyone know what to do with this problem. Its getting cold out here and i need the heat to stay warm.

I'm guessing a stopped up heater core...

could be a vacuum actuator not allowing the warm air from the heater core to come into the passenger compartment.