How do you open the trunk of our Mercedes 2000 S500 when the battery is dead The key doesn't work?

I had the same problem, the battery was dead and the key wouldn't open the trunk.

Here is an easy solution that works great.

Open the hood and locate the fuse box on the drivers side.

Remove a larger fuse such as 30A and insert a small piece of wire and replace the fuse so contact is made.

Set a charger to a low amperage such as 10A.

Attach the positive connection of a charger to the wire and the negative to the frame of the car.

This will provide enough power to the system to use the trunk release.