1999 Mercu. Sable – HVAC stuck on hot

Hi!
I need some advice on an HVAC issue with a 1999 Mercury Sable. It’s an LS so it has a climate control system (you set a temp rather than just a cold/hot knob)

I was driving on a nice warm day with the AC on, and all of a sudden it changed to full-hot and will not change back to AC.

I got home and tinkered, and found out the blend door is stuck on hot air. I took out the blend door actuator so now I can flip it back and forth manually (at least I got to have sweet cold air back!)

So I isolated the problem to the blend door actuator stuck on Hot. I got a replacement from Amazon, but this one does not move either (I knew that could happen) 
I assume at this point that the problem is in the component that controls the blend door act. either the whole climate control module or it’s thermostat. Is that a reasonable assumption?
In other words, what should be my next step? Can you service those units, and are there any flaws in my previous diagnostic steps?
Thanks in advance,
Gil