There are five actuators mounted on the HVAC Plenum that control various a/c-heater functions.# 1 actuator controls the defrroster,#2 actuator controls the air inlet door,#3 actuator controls the air mix door #2 passenger side,#4 actuator controls the a/c door and #5 actuator controls the air mix door #1 passenger side.
I beleive the last item is not functioning since you indicated that heated air is being emitted from the passenger side, but not from the drivers side.There are three actuators located on the left side of the HVAC Plenum. The one I believe that is causing your problem is the center one
Simple diagnosis would be is to have someone work the various a/c heater controls on the face panel while you visually see if there is any movement on the actuator arm on the drivers side acuator that is attached to the plenum.

Does this seem logical? Worth investigating
jb