Why is heater stuck on defrost in 98 mercury mystique ls?

If the heater in your 1998 Mercury Mystique LS is stuck on defrost, it may be due to a malfunctioning blend door actuator, which controls air distribution between the defrost, heat, and A/C. Another possible cause could be a vacuum leak in the system, affecting the operation of the HVAC controls. Additionally, a faulty climate control module or a stuck blend door could also contribute to this issue. Checking these components can help identify and resolve the problem.