Before you go digging and breaking into things and ordering repair parts thinking there is something wrong with your A/C system's damper door servo units, I suggest that you fully understand how the system is to supply heat into the cabin...and at what levels the upper outlet vents supply conditioned air versus the lower outlet vents - based on the cabin's ambient temperature, the outside temperature and whether cooling or heat is being called for by the system's temperature control settings. If you want to deviate from how the air outlet vents work in automatic mode you can choose which outlets you prefer. Refer to your Owners Manual about how to change them.

The manual vent control of some older and less-automatic HVAC vehicle systems (sample image below) allow you to twist a **** to choose which outlet vents supply the heat or cooling to the driver/occupants. You can choose for cooling to come out of the floor outlet vents or heat to come out of the upper outlet vents, both of which is not the best way to direct the air. The best way to distribute cool air is primarily through the upper outlet vents to the upper body...and the best way to distribute warm air is primarily through the lower outlet vents to the lower body, both of which is the way fully automatic HVAC systems direct the air.