I got a 2005 CLK Convertible, just changed a stepper motor (behind the ignition unit) because of a clicking noise when changing the direction of the air flow. I changed the stepper motor because of the clicking sound but also because I thought it may be related to the the top dash / windshield vents, I can hear it blowing at full speed but don't feel anything, works in all the different directions but cannot feel anything at the windshield.

What could be the problem ? No idea what to look for, doesn't make any weird sounds just doesn't blow to defrost the windshield.

I would get a scan tool and pull codes from the ACC module. A generic OBDII reader won't do it - you'll need one that reads MBZ proprietary systems. of course MBZ's SDS (Star Diagnosis) is best, but you can also use some others like the iCarsoft MBII or i980, or the Autel MD802 Elite.

Found the culprit : the stepper motor that controls the windshield vents was stuck by a broken teeth of the plastic gear. Replaced with a new unit.

I have the same problem in my 2005 Benz CLK. Was it complicated to fix?