Join the mercedes owners forum
http://forums.mercedesclub.org.uk/index.php
Search on there, or ask in a new thread, and you'll find how to input a live connection through earth points and a live terminal on your car, to provide sufficient energy to operate the door locks.
Quite simple, you just need to know where the connection points are. They are a friendly lot on the forum in the main.
Chicks Fan, August 2010