One of the many things that I love about the 123's is how well they come apart and go back together. On these cars this does not stop at the door panels. There are many cars, including other MB's on which taking off the door panels risks total destruction of them.
To remove a 123 door panel:
Remove the lock button.
Remove the two screws for the chrome trim at the striker plate and then the plate.
Pry loose the black plastic trim behind the door handle.
Remove the one phillips screw behind that black plastic trim.
Remove the large phillips screws at the bottom of the armrest.
Pry loose the chrome trim at the top of the arm rest handle.
Remove the large phillips screw behind that chrome trim plate.
The whole arm rest should now come off.
Look around for anything else that may hold the panel in place in case I have forgotten something.
Push a screwdriver between the panel and door sheetmetal around the bottom and pry loose the pop on fasteners.
Once the panel is loose at the bottom and feels like it is "hinged" at the very top, grab the bottom of the panel and pull up to loosen the top of the panel from the top of the door.
Be careful as possible to keep the plastic underliner intact. You will only have to pry up the lower rear corner on each door to access the vacuum diaphragms.
For replacement, start the panel over the lock rod and hook it on the top of the door front to rear.
Start the lock button in place to hold the lock rod, so you don't lose it.
Reverse the order of the disassembly steps.