Why does it still blow cold air when the heat is on, I have changed the thermost on 1998 GMC Jimmy

I also flushed the heater core and both hose get hot so not clogged, but still blows cold air, can it be anything other than blend door motor, and what would be the fix for any of the issues that it might be, P.S. its still cold in Wisconsin

Asked by for the 1998 GMC Jimmy