I have a faulty window regulator in my 1972 Mercedes 280se 4.5, and decided to remove the door panel to figure out how to replace it. It wasn't a straight forward process and so I took some photos of the location of the plastic clips so that if you're doing the same job, you have a point of reference to where they are and what they look like.
Firstly, I used Gerry Van Zandt's excellent description of how the W108/W109 door panels are removed. These instructions are applicable to most all older Mercedes, and I have added to them from my experience doing this.
1. Roll the window down (I had it stuck half-way up, so that was OK too)
2. Remove the armrest & door handle assembly via the screws holding them on. There are two large Philips screws on the bottom of the armrest, and screw at the top of the armrest is permanently attached. Remove it by rotating the entire armrest counter-clockwise.
3. 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. Basically, there are two pieces. The black plastic piece, and the metal chrome piece. Remove the black plastic piece by very carefully running a small screwdriver along the edge. Then you will see a Philips screw for the chrome piece, remove that too.
4. Unscrew the door lock knob.
5. 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.
6. 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.
What I did was take a very thin long metal plate (that you typically use to pix epoxy etc) and used as a lever to get the pins out one by one.
See the attached photo for the idea of where the hooks are and what they look like.
7. 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.
8. The front side of the door pocket is mounted using a metal clip that you need to slide the panel up to release. See the attached photo for what the clip looks like. After you do that, the entire panel should be off.
9. 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.
I hope this is helpful information to the forum. Good luck in your engineering efforts