Um, at least in my car the terminals are not dead. I can add a charger or an external battery and this source will power the complete car. I believe that I could even start the engine, if external power was strong enough. For opening the magnetic locks of the trunk, a small battery as that in the link I had posted is good enough.
So I still do not understand if it's a different configuration in US cars or if there is another reason why I do not have any problems with the trunk (and at least one other German E89 driver either -
http://bit.ly/1mNLpYG). Anyways, it appears that not all German cars can be opened the way it works with mine -
http://bit.ly/1mNLL1h.