If you're running high flow cats, or no cats on a race application, it is recommended to change your cabin air filter more often than the every b service.
Changing the air filters, like the spark plugs, is a little harder than on a toyota. Instead of just going behind the glove box, you have to go under and then back up like the plot line of a 1990's Arnold movie.
Under the glove box there are three T25 screws, these are held in place by little metal things that attach to plastic (they can be slightly misaligned upon reinstall which is why I am foreboding with this information).
You then pull up the carpet toward you and bend the plastic piece away from two plastic pieces one on the footwell area and one next to the door where the black meets.
Then you pull the black piece under the glove box aft and down.
There are two wires which are fairly fragile and paradoxically some how also difficult to remove. I have no idea what one of them is for, and the other is for a light. I have broken the mysterious side, and nothing has happened so I guess I'll reattach the wires back into the connector at some point.
The Back Up
Once you have ventured the under it is time to look back up and then remove the filter by sliding a white clip to an area that there are no more retaining tabs. The filter pulls downward. A new Mercedes cabin filter fits in one direction. The tab starts in the opposite direction for reinsertion. From where there is no retaining clip then slide over to where they overlap and the clip now holds the filter.
Before you put everything back in reverse order add some felt treatment on the plastic on plastic areas to reduce rattling. I also opted to add a bit of dynamat on the pieces that exhibited undamped harmonics when tapped.
Check out this write up on MBworld. Seems somewhat deprecated and without pictures, but may be useful: https://mbworld.org/how-tos/a/mercedes-benz-e-class-how-to-replace-cabin-air-filter-388754