Thanks for the answers guys! That really helped.
Now that I have an idea what the thing looks like, I think it'll be a lot easier to find it.
But until I find it and install a switch, the solar charger may be a solution (wiring my be a bit tricky though. If I remember it correctly from my earlier E320 W210, the cigarette lighter can't be used as a connecting point because it isn't connected when the ignition switch is in the "off" position).
If you can prove that you are the owner of the car, there is no valid reason for them not to let you know where it is. When the car is sold, they can always relocate it to a differet location if the new owner wants to continue the service. They even suggest me to pay them something like a $100 check-up fee each time I disconnect the battery and reconnect it, to make sure the device is working properly. What a ripoff!