No heat, full coolant

My coolant is full in both the resevoir and the radiator. Heat worked fine one day, then the next did not. Nothing but cool air comes out. I felt the hoses going into the heater core and both are hot (meaning there is fluid circulating through the core). I have half of the dash apart, but still cannot find where the flap would be to switch it over to heat (assuming the hot - cold VR knob is not functioning). Also, the temp guage is a bit irradic. Any ideas?