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If the heat works great on the drives side, but not the passenger side, then the problem is either the blend door actuator or the actual plastic door itself in the evaporator case. You can remove the actuator and using a large flat blade screwdriver, turn the door manually and see if you can manually control the heat and cold air for the passenger side. If you can manually control the air temperature, then replace the blend door motor for the passenger side. But if you cannot control the temperature, then you will have to replace the door and it is part of the evaporator case.
See the directions below for the actuator.