How do I remove the rear door on a 1989 Mercedes 300SE?

I've done this several times. You should probably disconnect the negative terminal on the battery to avoid shorts.

1. Unscrew and remove door locking knob.

2. Using small screw-driver, pry out plastic cover under door-opening lever. Remove securing screw underneath and pull out and disconnect the frame including the electric window switch. You will only be able to move it down the upper edge of the door handle enough to get at the large screen that holds the top of that handle.

3. Remove the top and bottom screws (large cross-type) that secure the door handle, and remove it, along with the frame in #2.

4. Remove the two screws retaining the chrome door latch moulding on the back edge of the door.

5. Remove the trailing Opera window bumper by removing the two small screws on the top rear edge of the window frame.

6. You can now remove the door panel by sliding it UP, while using a screwdriver to pry up the lower edge of the door-opening level a bit, so that it clears that assembly. Do not attempt to just pry it off like most other cars - you will irreparably break the retaining plastic bits.

7. Finally, you'll need to disconnect the ashtray light/lighter cable and the courtesy light before you can completely remove the panel.

Re-assembly is the reverse of the above (more or less). The most tricky part is sliding the panel back down onto the door while getting the door-locking post back into the hole and engaging all the plastic hooks (including those in the top edge) while clearing the door-opening-level assembly. This is likely to require several attempts and may be easier with 4 hands (or two people).

Note that the top plastic tabs are often already broken on older models, resulting in a fair bit of noise. If this is the case consider (gulp), drilling a hole through the middle top of the door into the door frame, at an appropriate place, and insert a large-headed but discreet screen. It will quiet things down considerably.

Before you replace the panel, consider lithium-greasing the various linkages exposed on the inside of the door, again to quiet possible rattling.

Lastly, whenever you remove and replace trim on the inside of these vehicles, run a cloth that is somewhat saturated with a silicone lubricant on all contact parts - do not spray directly to avoid a mess. This will also help eliminate squeaks and rattles.

Have fun!