Here's an opportunity for MB to improve their design...

The wider side of the seat hold two headrests, four holes for headrest posts. All four holes are evenly spaced. It's quite possible to stuff a headrest into the middle two, one post in a spot for the middle seat, one post in a spot for the outside seat. Don't ask how I know this...

The only solution is to remove the seatback from the car and partially disassmble it. The "cutout" on the headrest post is locked in, and there's no way to release it without opening up the seat and manually releasing. All four post holes have the same retaining mechanism, but only the ones with release buttons have a release mechanism installed.

It took me about an hour to fix this on my, umm, friends' car, yeah, it was my friends' car...