That little fan motor behind the passenger side air vent is called the aspirator motor.
It sucks in air and blows it up the A pillar, through the headliner, and out through the vent that is right next to your dome light / sunroof control button.
The temp sensor for your a/c is up there, which registers the temp for your wheel, and also blower speed.
My mechanic replaced mine two weeks ago. It looks like he inserted some thin plastic stick into the air vent and inside tab in the air vent on each side, pulled the tabs toward the middle, and the plastic vent pulled right out.
He could pull the aspirator motor right out so that we could see it and hear it run. Mine was noisy.
He simply unplugged it and installed a new one. Took all of 20 minutes.
He explained they go out all the time. He replaces them a lot on C classes, too. He had one from a C class on his desk, and it was tiny. They are just able to make parts smaller and smaller to save money, and weight.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".