The air delivery system in your Sonoma is vacuum controlled and uses engine vacuum to open and close doors in order to move the air output to the different vents. The default is air out of the defroster only. I would suspect you have a vacuum leak to the air switching valve assembly which is part of the heater and a/c (HVAC) on the dash. The most common location for the leak would be under the hood. There should be a vacuum hose from the engine that connects to another vacuum line that goes through the firewall, the vacuum lines will also tee off to a plastic vacuum tank which is located in the right front fender. Make sure all of these lines are connected and there are no leaks. If everything is OK under the hood you will need to check for proper vacuum at the HVAC control assembly.