Heat or AC dash vents inop

1197 Buick Lesabre with the climated controlled air / heater..
55000 miles v6 auto loaded.

I have a problem with only the vents considered mid on the panel. I cannot get any air either from heat or ac to come out of the dash vents.
It will allow me to either have air from the floor or the windshield area.
I am wondering what controls the flap or door for the dash vents. Is it electric or vacuum? The airflow indicator will stop at and shoe that the mid level is chosen but no air from them.
Any thoughts or ideas would be appreciated.
Thank you

Your HVAC controller has all the solenoids inside that operate the mode door positions- the purple vacuum hose that leads into it supplies the necessary vacuum source- many times the vacuum source has a leak underhood- where the line is black plastic- that goes from intake manifold to the vacuum reservoir then inside the conduit to go inside the car and under the dash-- check all these lines first for a vacuum leak- broken plastic line, or rotted rubber connections will leak enough to bnot allow vacuum doors to operate properly-- the doors need at least 10-15 In Hg to move against the spring loaded actuators-- so check vacuum supply first---

If the purple line under the (and behind) the glove box is supplying proper vacuum, then you probably have an internal vacuum leak att he HVAC controller module- at the port entrance-- I have seen two like this-- I ended up bypassing all the porting and running soft rubber hoses to the and from the HVAC solenoids- this solved the problem... That HVAC controller is like $900 new-- salvage yards have them for $125-- but the last one I got from a salvage yard acted the exact same way as the one in the car--as rubber and plastic get old- it leaks and cracks- and vacuum leaks occur-- you need to understand where your leak is-- with lots of car idling and turning down the radio and you can hear the hissing, leaking vacuum lines-- good luck...

Thanks for the reply. I am still lost now. You say the hvac controller. Do you mean the controller in the dash I adjust for temp and air flow.
If so this has no vacuum plumbing attached to it.
Yes the is a junction behind the glove box that has about 5 vacuum hoses to it.
Is a controller somewhere in that area i am missing to find?
Thank you

Terminology--

The control head is in the dash- has all the input buttons

The HVAC controller is mounted on the side of the HVAC module- behind the glove box- it has the pretty vacuum lines inputs and outputs-- it is what I am referring to..........look/listen for a vacuum leak

If you've got vacuum coming into the interior that will move the doors to actuate the floor only setting and floor and defroster, you most likely have the problem where the line for the dash vents has a black plastic that blocks it where it fits into this connector.

http://i86.photobucket.com/albums/k110/imidazol97/F2-open_programmer.jpg

You can check to be sure the tube is on the vacuum motor that moves the doors inside the heater box.

Otherwise you can cut out the plastic connector and use small 1/8 inch gas line pieces to connect internal and external tubes or there is an aquarium hose that's the right size, I'm told.

The programmer box which may or may not come with this connector is $439 from rockauto.com #16258434.

Thanks to all that answered. I took the HVAC unit out from behind the dash and took the cover off of it and looked at the inside and sprayed some cleaner into the vacuum manifols. Plugged it all back in and it now works. I am wondering if the bumping off removeing it jarred the diaphrams inside the solinoids or if the cleaner lubed the rubbers up a little allowing them to move freely.
BUT the good news is it is now working. I have nice cold air coming out of the dash vents...
Again thanks to all that posted.