Why wont the dash vents work in your 99 mercury sable but the floor vent and defrost vent work fine?

The issue with the dash vents not working in your 1999 Mercury Sable, while the floor and defrost vents function properly, is likely due to a malfunctioning blend door or a problem with the HVAC control system. The blend door controls the airflow direction, and if it is stuck or damaged, it may prevent air from reaching the dash vents. Additionally, there could be a vacuum leak affecting the actuator that directs airflow. It's advisable to have a professional inspect the HVAC system to diagnose and fix the issue.