Defrost is the default if nothing else works. You are not getting vacuum to the pod that directs air flow elsewhere or the rubber diaphrams are cracked within the pods.
If you remove the right cover of the center console, you will see a row of vacuum relays. These switch vacuum to the different pods. You can use a gauge to check vacuum input to the stack and then from each relay. This is how you can determine which pod needs to be overhauled. You can buy the replacement inserts from
here