Mounted to the HVAC box behind the dash are a series of small electric motors called "actuators" they open and close the doors as need to control heat/Ac, vents etc.. in most cases when you lose a side like you have, a blend door actuator has failed. the one for drivers side would be the bad one. It is #7 in the picture. the problem is accesibility. In todays vehicles with this design, most cases you have to remve the dash to get to the actuators. this is the case with your truck. I put directiosn below. the dash has to come out to access the actuator and replace it. Now.. a shop with scanner can hook up and view data on the actuators to be sure they work or not. 99% of the time, the actuator has failed. Another option would be the control panel but agian, a scanner is needed to confirm. Book time for the actuator is 4.6 hours to change it. It is a big job. still unsure why manufacturers put them where they did and make so hard to get to them,. just bad design.

REMOVAL
The single zone heating and air conditioning system is equipped with a single blend door actuator. The dual zone system has two blend door actuators, one for the driver side blend air door and one for the passenger side blend air door. The same service procedures can be used for each of these actuators.

WARNING: ON VEHICLES EQUIPPED WITH AIRBAGS, DISABLE THE AIRBAG SYSTEM BEFORE ATTEMPTING ANY STEERING WHEEL, STEERING COLUMN, OR INSTRUMENT PANEL COMPONENT DIAGNOSIS OR SERVICE. DISCONNECT AND ISOLATE THE BATTERY NEGATIVE (GROUND) CABLE, THEN WAIT TWO MINUTES FOR THE AIRBAG SYSTEM CAPACITOR TO DISCHARGE BEFORE PERFORMING FURTHER DIAGNOSIS OR SERVICE. THIS IS THE ONLY SURE WAY TO DISABLE THE AIRBAG SYSTEM. FAILURE TO TAKE THE PROPER PRECAUTIONS COULD RESULT IN ACCIDENTAL AIRBAG DEPLOYMENT AND POSSIBLE PERSONAL INJURY.

1. Disconnect and isolate the battery negative cable.
2. Remove the instrument panel assembly.

1. Disconnect the HVAC wire harness connector for the blend door actuator from the actuator connector receptacle..
2. Remove the screws that secure the blend door actuator to the distribution housing.
3. Remove the blend door actuator from the distribution housing.

INSTALLATION

1. Position the blend door actuator into the heater/air conditioner housing. If necessary rotate the actuator slightly to align the splines on the actuator output shaft with those in the blend air door linkage.
2. Install and tighten the three screws that secure the blend door actuator to the distribution housing. Tighten the screws to 2 Nm (17 in. lbs.) .
3. Reconnect the HVAC wire harness connector for the blend door actuator to the actuator connector receptacle.
4. Install the instrument panel assembly.
5. Reconnect the battery negative cable.
6. Perform the heater-A/C control calibration procedure.