Mercedes certainly hid this one! Had me puzzled too until I finally figured it out.

Do *not* try to pull the back panel free - unless the clips are badly sprung you will probably crack the plastic panel. Instead, look for the channel in which the side upholstery is secured into the rear of the seat. You should see 2 silver clips (per side) that secure the upholstery and back panel to the seatback. Carefully pry out the clips and everything will fall-out-of-place.

Installation is obvious, now that you know how they did it.
Good luck, Barrie