I removed the panel on my 450sl some time ago. I think it should be the same. 1. remove the chrome trim pieces from the door inside. 2. remove the handle and open latch. 3. pop the plastic buttons from their holder holes along the bottom of the door by unsing a flat tip screwdriver and pying outward carefully behind the vinyl trim. 4. pull up ward gently to release the top of panel and holding clips and the panel should be free.
The only pace you may get damage is the plastic buttons along the bottom of the door trim if you pry too hard. The masonite backing might pull through if it has been weathered.