How can I open the trunck 2001 Mercedes s500 if the key it's inside of the thunk and my battery it's flat

Open the hood on the inner fender near the bulkhead you will find a plastic box about the size of a matchbox with a red '+' on it. The top will flip up to expose a terminal, connect your Positive jump lead here, connect your negative jump lead to an engine earth. CAUTION !!! before connecting to jump battery , ensure that you leave a door open as powering up may trigger the alarm or central locking system!!. Once connected to a junp battery / vehicle you should have power to operate the trunk release button on the console.

Paul there isn't a boosting terminal under the hood of this vehicle either. The only way to power up the main battery circuit (MB calls it circuit 30) on a W220 is to open one of the fuse boxes under the hood. There's one on either side near the rear. You'll have to remove a fuse and jump from a separate battery to the power side of the fuse and ground of course to the body or engine block.

Fine the alt and remove the cap on the + cable and connect your power to that and open the truck . Do not try to start the car, you may burn the radio up.