I have an 83 380sl and just went through the same thing-here's what I found (but I can't vouch for if they all work this way).

On mine, the speaker covers had two small screws holding the bottom down and trim glue holding the top on. I know my speakers are not original so I can't vouch for the "factoryness" of this. Once you get the covers off, replacing the speaker is easy-you may have a hard time finding speakers that are an exact fit, but you can trim the excess meat off the speaker mounts and you will be fine.

I was a little surprised that the grilles mounted the way they did, so please don't take my word for it 100%.

Good luck..

J. Boggs