Ok after reading the pdf you linked its going to kill power to the alternator when the keys off too so checking that wire isnt going to help. This means you could have a bad fusible link OR a bad relay in there too. Im not familiar with how Mercedes does things but on my bmw's they use an explosive charge to disconnect one of the battery cable leads at the battery itself in the event of a wreck. Im guessing there is something similar to that inside that unit. If you havent had a wreck I would imagine the relay has gone bad that the ignition switch operates if the problem is inside that.