Most likely cause for HVAC system to default to defrost is a leak in one of the hubs.

Diagram of ESOF vac lines.





Remove one and plug it. Will HVAC come off defrost? It can take a few seconds. If yes that is the leaking hub. If not try the other side. With both vac lines plugeed and it is still defaulted to defrost check vac system under hood. Make sure all vac lines are tightly connected.



Another way to isolate leak to hubs is follow the hub vac line to a 90 degree elbow fitting. Pull the vac line going back to the vac reservoir and plug it. If the HVAC comes off defrost one of the vac lines or hubs has a leak.

Also, if the pump doesn't run there will be no vacuum for the HVAC system and it will default to defrost.
1999-2001 Under Dash fuse #28 is for the vac pump among other things.

Edit: A leak in the vac reservoir tank will also cause default to defrost.