The stock speakers are "clamped" in the dash. If you look at the topside of the speaker, you should see the ends of the three clamps of the speaker.
You have to start at a clamp to get the speaker out (push the clamp towards the middle of the speaker).
Take care; the speakers can be very hard to get out (they are clamped in very well) and if you finally get the speaker released it can hit the inner side of the windscreen (better be safe as sorry).

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)