will not warm up

What is your question?

It's that time of year up here, where everything is getting colder and you'd want a nice warm rig to get in to when you get out side. here's my problem: i replaced my thermostat, everything works great in the summer, but around fall/winter my rig just doesn't want to blow hot air. it will just get by on defrosting the windows but wont warm up the whole car. blower works just fine, just not hot air. heats up great during the summer to the point where i drive around with the windows down almost all the time. is it the heater core? are my resistors shot?