I don't know if this applies in your situation, but...sometimes if you remove the upper section of the back seat, there is a fiberboard panel in the middle of the seat support sheetmetal than can be pushed into the trunk. Had a Volvo with this don't know about Mercedes, but it won't hurt to look. The kicker is the opening was a really tight fit for an adult, so if you do have one, you'll need a small adult (try Wal-Mart), or a really intelligent,mechanically inclined child to get in there. Seriously, if you can get 1/8 of an inch, try a piece of paper and see if you can feel if part of the locking mechanism is still engaged on the latch. If your paper will not pass the area of the lock, then you could try the old credit card in the door trick. A reasonably thick piece of plastic. You'll have to push and wiggle it around, but who knows, maybe you'll get lucky. Otherwise, you may have to get the lock cylinder either drilled or pulled out with a slide hammer. Either way, this doesn't sound like fun. Hope any of this helps you.