Air only blows out the defrost vent

This is the anniversary edition correct? Does it have the automatic or manual control system. They are slightly different. But the most common issue is a broken vacuum line or bad vacuum control from the control head.
If it is a bad vacuum actuator the heater box has to come out to get to it. That means the instrument panel has to come out, the AC and heater system has to be drained and discharged, then you can get to the controls that are on top of the HVAC module behind the dash.
It is a real bear to do so I hope the issue is a line.

Hello,

Yep that is the controller that adjusts the temperature and air flow or the system. Use must fix the arm or replace the unit.

Please let us know what happens.

Cheers, Ken

As Ken said, that is a linkage arm. The ridges are basically to lock the plastic on. Set the temperature to full cold, now move the arm until you get full cold air and the door stops. That is the point you clip the two parts together. If they have fallen apart (usually because of the age of the plastic) you can use a couple small zip ties or even thin wire to hold them together.

Can you please take a picture and upload it so we can see what you are talking about?