why does air only exit through the dash vents only. on 1992 Mercury Grand Marquis

a tried to change from floor to defrost. airflow still will not change

I would also check the heater box to see if it is cracked or broken causing blend door not to operate properly
1 more answer , 1 more comment
Your air ducts are not connected. Look under your dash and re connect them. If the connectors or snaps are broken, duct tape the ducts to the vent windows. Good Luck.
Most older vehicle have the air outlets controlled by vacuum. I would look for a vacuum line going between engine and fire wall and see if what condition it is in. With engine running take a connector on the line apart and see if you can feel a vacuum. If you have vacuum before fire wall, go under dash and look for continuation of line to se if it is intact. Also on older cars the dash control can fail and will no longer supply vacuum to item that you want to control.