Natalie,

Most of the time trunks are not opened with keys, since they are automatically locked, so the locks get stuck. The way to open the trunk is to remove the hose from the pump that activates the lock by pressure. Under the rear seat at the right hand site this pump is found. There are two latches underneath the seat that you need to press to remove it at the corners. When you see the pump there are some hoses connected to it...one of them states "heckklap" =trunk in German. Simply remove the hose and use a compressor to put some pressure on it...fi. at a gasstation.
The compessor looks like this (at least on my W210).

Cheers,
Ingmar

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