Gerry Van Zandt wrote this very clear description of how the W108/W109 door panels are removed. But these instructions are applicable to most all older Mercedes.
Roll the window down entirely into the door . Remove the armrest & door handle assembly via the screws holding them on. Be careful in removing the molding immediately behind the door opening handle. Use a tiny screwdriver in the small slot at the edge of the plastic piece behind the interior door handle to pry it off, then you will see the screw in the chrome piece behind it. Unscrew the door lock knob. Remove the small Phillips screws on the outer edges of the door panel, and the one on the inner door panel just above the rubber air passage seal. Now comes the hard part. The panel is held on several plastic hooks, which slide INTO the door then DOWNWARD when the panel is moved into place. It is EXTREMELY easy to break these hooks off. The door is attached pretty tightly across the top where the window goes down. You need to CAREFULLY apply pressure upward and kind of outward to slide the hooks out of the slots. If this is the first time you have done this, despite using extra care you will probably break off one of these hooks. This is not the end of the world. The door will still go back together just fine. Carefully remove (or fold back) the plastic sheet adhered to the inside of the door without ripping it. When you replace it, you may want to dab with a bit or contact or rubber cement to put it into place. Once you are inside the door, check for the following by moving back and forth for rattles:
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