The air vent problem is constant. It always get to default mode (cooling windshield) as soon as the car is started. When the car is running at night with very little surrounding noises, I could hear the sizzle sound under the dash. I think the vacuum connection may be bad at the actuators area. I had tried to look into the problem by opened the side panel under the glove box, but couldn't reach the inside to see. I was too cautious to poke further before I understand the structure of the under dash. I had the tendency to screw things up worse everytime I did something, and caused extra works. May be I should start to remove the radio and look from there. I need to learn the procedure to get the radio panel removed though !
Also the air flow completely sent to the windshield during aceleration. Air will get back to the side vents after 2 to 3 seconds of leveled speed. This has been a pain in the you know where when the hot temperature in central Florida climb to 98-100 degrees in the summer.
The MB dealer has done the repair on this before when the car was under warranty and the control pushbuttons did not even change the air flow as you wanted it to, i.e. to the floor, or to the windshield.
Hope to get these mistery problems fixed once and for all.