1999 Cadillac El Dorado dual climate control

Open the glove box and remove contents. If there is an access door for the passenger airbag connector, open it and you should see a linkage rod clipped to an actuator arm. I believe this is the passenger temp door actuator. Unclip the rod from the white, round clip and move it by hand to see if temp gets hot. Move the heat controls to see if the actuator arm moves at all. Either you have a linkage rod that is disconnected, or you have a faulty actuator.