You need to do the draw test to find out what circuit is at fault. With the amp meter connected in series on the negative battery cable check the draw. You need toconnect your meter to the negative battery and to the negative terminal on the battery before fully removing the cable from the battery. After doing this wait for about an hour for all the computers in the car to power down. If after an hour wait you have more than a 50 miliamp draw you need to start reomving fuses from the fuse boxes one at a time while watching the meter. When the draw goes down you have found the circuit you need to check.
Mercedes Benz Master Tech