1997 Mercedes C230 center vent is not blowing air (cool or hot). However, the remaining vents (side/top/etc.) continue doing their job. The air blows so well from the other vents that the front windshield often becomes foggy when the a/c is operating. What needs to be done to troubleshoot this issue?

 

Sounds like a center flap is faulty. To confirm that you would need to do a flap test to confirm or find someone with DAS to do the work. Search the web for MB flap test procedure.

 

I have the same issue in my 2000 C280, got it quoted around 300-350 I believe to fix. I can't recall what the mechanic said the issue was however, probably a leak.