1994 mercury capri heater is not working. The water temp gauge stay cold and i changed the thermostat. How can i check if the blend door is not working?

To check if the blend door is functioning, first, locate the blend door actuator, usually found near the heater core. With the vehicle running and the heater set to hot, listen for any sounds from the actuator; if it's not moving or making noise, it may be faulty. You can also manually move the blend door if accessible to see if it opens and closes properly. Additionally, check for any vacuum leaks or electrical issues related to the actuator.