I just did it a few days ago. Pull up the floor carpet first, it just unsnaps. Then remove the screw mentioned above, and the three screws on the front of the panel just under the glovebox. Remove the door-side trim next, it pops off. Then open the passenger door and pull down the piece under the glovebox. It moves down and towards the right (out the door) and may take a little jiggling to disconnect it from the vent hoses and so on.
I also removed the glovebox interior when I reinstalled the piece so I could be sure that I reattached the hoses properly.
1988 420SEL 170K