Heater Controls

I have dual temperature controls. A couple of years ago I could not get air to come out of the vents. That is fine. I could still get cold air out of the defrost and floor. Then last winter I could not control the heat. Once I got it to a fairly warm temperature. I just left it. Today I had the air conditioner on and it blows hot out of the drivers side and cold out of the passengers side. The parts dealer tells me there are three different actuators, and wants to know which one is bad? I am not sure, nor am I sure where to access the one that I think is bad.