Your AC is controlled by engine vacuum, so if you have a vacuum leak you won't be able to change the output, and the default (no vacuum) setting is defrost.
Look for a vacuum leak starting with a white vacuum line that comes out of the firewall by the heater hoses and ac lines trace that one.
Additionally, there is a vac line that runs behind the battery. It tends to rub on the battery which puts a hole in it.