Broke Heat Control This Morning - What Could It Be?

Well I got up this morning at 6:15 am in Northern Michigan and it was pretty cold out. Decided to start the car up then go back in to get ready for work. Started the car and went to adjust my heat (blend door). It had a little resistance to it so I backed it off a little seeing if it would free up. Well it didn't.. I ended up breaking something inside. It took little effort to break it. So I was like.. Motherf#%! I drove to work with no heat. Now I have to take the heater unit out and investigate. My first thought was that I broke the knob. However, it just turns as if nothing is attached on the back side.

Back in September, I replaced the heater core. I remember seeing a gear on the back side of the unit and teeth on the cable. I am assuming one of those broke. It if is the cable, how hard are these to change out? Does the dash need to be pulled?

Just went out on my lunch break and was able to remove the knob. What is odd is that this morning I pulled and pulled on this thing and it refused to come off. It just slipped right off now. So now I need to find out why it broke. It shouldn't have that much resistance.

Thanks for the heads up. The knob appeared to be glued on this morning. I was surprised it wouldn't come off yet it was so easy to remove at lunch. I will have to take a look at things..