First of all, welcome to the forum....basically a bunch of knowledge from a good group of folks.
As far as your heater, the next place to look is the heat mix door. You probably know this, but that's the flapper that mixes hot air going through the heater core and cool outside air. Sounds like it's either stuck and not moving at all, or there is something stuck in it not allowing it to get just hot air. If the heat control knob changes the temperature coming out of the vents at all, the flapper door is moving and I'd think there is probably something stuck in it. I'm not sure if you can reach the flapper door from the cabin filter behind the glove box or not, but that's where I'd start.
Perhaps someone else can chime in that knows where the best access place is.