I can't speak from experience, but if it's anything like the W124 (fortunately MBZ tends to follow similar methodology even as their designs evolve), you will have to remove the whole bumper and remove the impact strip from behind. Removing the bumper is typically not a big deal - usually 4 or 6 bolts removed from inside the trunk area.

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280