97 C230 AC Problem
 

I just purchased a 97 C230 with 130K. I have a problem with the AC. When driving it changes from vent to defrost to only blowing from the two side vents and nothing coming from the to middle vents. could it be a vaccum or is it the AC head its self? I heard the AC head is quiet a bit of cash! Is this a common problem for these models? Does anybody know?? Thanks