Yes it does it with the cold air too, does anyone know what this part is called

Hello, I have a 2005 vibe it is not in warrenty anymore and this just started to happen (of course when it is out of warrenty) The air an heat in my car only works when it is full blast, Does any one know why? What do I need to do to get it replaced? How much do you think it will cost. I just had th...