Start by taking out the rear seat. The back rest is mounted with three screws to the back panel.
Then take out the rear head rests by clicking out the plastic rear deck covers and rotating the clips (finding the clips to unhook them needs some attention; they are located underneath the deck in the plastic part of the rear head rest holder) and pulling the head rests.
Take out the rear side panels (yours is a 1996 E-class which are fairly easy to take out) starting at the door side.
Take out the rear deck...
Maybe someone else can give you more detailed info, but this is about the route to go...
with a minor 600+k