2006 Mercury Marquis Cannot Get Direction Registers Move Defrost And Floor

I cannot get the direction registers to move , defrost and defrost &floor works. I have taken out the radio and checked for vacumn leaks inside and found none. How do I determine what the problem is? This is a LS so the climate control is auto
The engine seems to run fine with no noticable problems. A slight vibration at idle nothing else
What to do???