Hello dsturbrud, look under the hood for a broken vacuum line. The system needs engine vacuum to control the mode doors and when a loss of vacuum occurs it defaults to the defroster ducts. Look for a black vacuum hose to exit the firewall and follow it to the engine. Look on your catylist sticker and look for the line that says to HVAC, that will help you locate it. You most likely have a broken or degraded hose.
Your JA Expert,
Ivan
ASE Certified Technician
ASE Tech. 18 years experience. 9 years self employed. Expert on AUTOLAB Radio Show