1999 Dodge Ram AC switching

I have a 1997 and had the same problem and believe it or not it was a pin hole leak in one of the tiny black hard hoses going into the firewall from the engine on the pass. Side. The small hose is a vaccum that goes directly to your climate control.

Check the vacuum lines from the engine to the firewall. See if the hoses are cracked or disconnected.
You have a vacuum leak and should hear a hissing noise.
The system is designed to go to defrost when there is a lose of vacuum.
Let us know what you find.