If you have to remove the rear deck, take a look at this tutorial. It is in French and pertains to adding a rear electric roller blind but it shows how to remove the rear shelf:

http://mercedes-benz.forumactif.com/Les-classes-C-c5/W202-f18/TUTO-Pose-d-un-Rideau-electrique-sur-plage-arriere-56ko-t7134.htm