Most likely the door holder has jammed up. you may be able to shut the door with one good shove (kind of like a limp-home mode for when it happens in the supermarket car park) but it won't last. It'll need replacing.

Here's how:

http://www.geocities.com/kerjagrafis/door_holder.pdf

http://www.geocities.com/kerjagrafis/door_panel.pdf

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1987 300e manual 250,000 km (sold)