I have 99 Limited with the digital display unit. Originally when searched the web for the issue, I was thinking that it was the unit that was bad. However, I will say that all the buttons and control knobs work and I do see a response from the display. Meaning when I push the AC button it kicks on and the lights turn on, cold air comes out. When I turn the knobs the temperature and airflow increase or decrease accordingly.
Now here's the tricky part, when I select defrost or try changing the modes of airflow from (driver )high to (feet) low/ both I see the display change accordingly. But, there is no change in where the air is coming from. Is it possible there is a bad solenoid or control module that tells the vents to move accordingly?

On a side note i picked up a used climate control module from the local pick and pull. That unit barely works, just the back light.

Search this forum (better search option is in the stickies). You'll find Both someone who'll repair your unit for you and instructions on how to repair it yourself if you're handy with a soldering iron.

Best I can tell, it's simply due to cold-solders that break over time and it needs to be reflowed, but I paid the forum member to fix mine (very happy with that interaction).

It's not the control. Either you have a bad servo motor or the the damper doors themselves are stuck. What vent is the air coming from right now?

That's kinda what I'm thinking due to all other buttons working with no issues. The air is coming out of both the upper and lower. Not really an issue but defrost doesn't come on.

I can't seem to pin down the location of the door or solenoid that controls the door/vent.

It's called the mode door actuator or air outlet servo motor. It looks like it's on top of the heater box and the book time to change it is 5.3 hours, meaning your whole dash has to come out.

Wow , thank you for the heads up.