Why are you only getting air out of defroster on 1989 mercury topaz?

If you're only getting air out of the defroster on a 1989 Mercury Topaz, it may be due to a malfunctioning blend door actuator or a vacuum leak in the HVAC system. The blend door controls airflow between the different vents, and if it's stuck in the defrost position, you won't get air from other vents. Additionally, a clogged heater core or low coolant levels could also affect airflow. It's best to inspect these components to diagnose the issue accurately.