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.
next to the spare tire in the trunk.
Does it not have a key lock on the trunk lid.
In the Trunk.
The battery on a 2000 E Class is located in the trunk. Mercedes puts the battery there so there is no chance of it being ruptured in an accident. The new E Class battery is however located under the hood just in front of the firewall. I think the above answer is wrong. The battery of the 2000 E class is under the rear seat cushion. Push up on the clips at each end of the rear seat cushion and unclip it, the battery is then found on the passenger side under the seat.
in the trunk
2000-2004, the battery is in the right front fender. 2005-2014, the battery is in the trunk.
The battery is in the trunk on the passenger side under a plastic tray.
In the trunk next to the spare tire.
In the trunk, passenger side
In the trunk next to spare tire
remove the trunk floor and the battery is on the right next to the spare tire.
coz its a Mercedes and you need a rolesroice phantom. also just get a new battery