Disconnect the large wire at the alternator and see if the draw goes away. ( the big one that leads to the battery) A shorted diode in the bridge will do this, my 97 C280 had same problem.

Also remove and check the small alt wire for power on the wire, it is switched power that tells the alternator to wake up. If the wiring has been hacked or a aftermarket radio installed / wires bridged, the alt can stay turned on. With the wire attached,key off, put something steel against the pulley and check for magnetism. If it sticks the alt is staying on.

A clamp on amp meter is very helpful in finding drains.