I am saying that if the diode is shorted that the condition you have could exist. The diode is inside a black plastic shell. It is a little bigger than a two prong fuse and it has the same round connector pins as MB relays do. It is located at the back of the relay section of the fuse box (requires removing screws to take off second cover). The absolute left rear corner.
As I said above it will look like a wire in both directions with an ohm meter. A proper diode will have high resistance one direction and low resistance the other direction. It acts as a gate alllowing current to flow in only one direction.
Bosch Master, ASE Master, L1
33 years MB technician