Inside of your heater module (under the dash), there's a plastic flap that's called a " Diverter door" and " Blend door".
Your unit has one Blend Door which controls the temp by mixing heated and cold air to get the right temp. I believe there are three diverter doors in your system. These doors directs the flow of air to whichever vents you have selected. (Floor, dash, defrost, mix)
They are all controlled by vacuum fed to them by black plastic lines. Sometimes these lines become cracked/loose, or the diverter door will stick.
You'll have to pull your heater control unit and have a look.