Inside the air box where heater core lives is the Blend Door, that is the most likely problem.
It is controlled by a motor(actuator), and that is usually what quits working, although the door itself can break where it attaches to the actuator.
To get to the actuator and blend door requires a bit of work in removing dashboard parts.
Google: ford ranger blend door actuator replacement
I didn't see any specifically for 2009 Ranger, but you will get the idea of what is involved and how to do the repair