It might be an easy fix, but it certainly wouldn't be something I would recommend for someone who has little or no knowledge of electronic climate control. The blend door, which regulates the amount of heat and cold mixture is controlled by an electronic actuator. Basically this is a small electronic servo that receives a signal from the electronic temperature control module. The electronic temperature control module gets it's signal from operator input and various sensors throughout the car.
Now, you have to decide what has failed. Is the blend door actuator not responding to inputs received from the electronic control module? Or has the module failed and not sending information to the blend door actuator? Or is the module receiving bogus information from a sensor and doesn't know to command the driver side blend door actuator to open or close?
And don't think for a moment this is something a person could test with a 12 volt test light. This is something a trained technician can diagnose with a scan tool and pinpoint testing. Yes you could get lucky and replace an actuator and have the problem fixed.
But if you have a wife like mine and you make a $100 misdiagnosis and the thing still doesn't work, you might find yourself sharing the dog house with Fido.