Airflow deflector stuck in defrost mode

My airflow deflector has become stuck in defrost mode. Where is the mechanism that operaties the airflow deflector, or how can I fix this problem. Thanks

Are we talking about the 1985 Buick? That is cable controlled. You may have a broken/seized cable or the doors seized. Just follow the cables under the passenger side of dash and check their operation. They should move freely. If not, unseize/lubricate them. Good Luck.