Weird Air Vents 300E

[sptt]

On my 91 300E, I selected the Defrost Selector and my vents will not return to front only, front bottom only, etc. The Defrost vent is stuck in the open position and air always blows from the front, front sides, and defrost vents. Prior to slecting the defrost, everything worked perfectly. Any ideas on pinpointing the issue? AC still blows ice cold through front and side vents but aslso through defrost vents. if I switch to defrost, hot air blows out of the defrost vents, side fron vents, but cold air blows through front middle vents. Also, I cannot stop outside air from coming in by selecting the recirculating switch ether.