2003 Cadillac heater/vent problem

My 2003 Cadillac DeVille will not switch between the floor vent and dash vents when running the AC.

Loud clicking sound from under the radio. What needs to be done and can I repair it myself? I have a mechanical background and am not worried about trying repairs myself if I know what part(s) are needed for repair…Paul

That clicking sound is likely the blend door motor trying to move a stuck blend door. Expect it to be difficult to get to to fix.