A/C air blowing out of defroster vents

Just recently the air flow when my a/c is on has directed itself to flow out of the defroster vents more than the vents in the middle of the dash of my '86 300SDL.

I had this problem with a '87 300D, and my mechanic (in another city now) made some repair or correction and didnt even charge me, he did have to go into the dash I believe or at least the upper console where the ATC unit and radio are held.

Any hints on what I might need to do or have done before I have any major work done?

Thanks,
Chris