Defrost is the default mode for your A/C modes, sounds like the vacuum control section of your A/C is not functioning.
First thing to check is the 12v vacuum pump, these are usually on the firewall near the heater hoses, and A/C Freon lines. Check to make sure it powers up with the ignition on.
Also look for a company name , for the A/C unit supplier " Evans " is common, cast into the plastic housing.
Knowing the brand will help if you need more repair info.
The vacuum pumps are available at AutoZone $85. if that's the problem.
If your A/C unit happens to be Acme. Then in the " Files " drop box ( Blue bar to the right of "User CP ") look in Air conditioning, there is a downloadable PDF file that may help.