Running again. I hooked up a voltage tester with an audible signal when a drain of >200ma was present between the negative battery terminal and cable. I then started removing fuses and relays hoping to hear the tone stop, indicating I'd found the culprit circuit. No luck and finally gave up and took the car to a local independent MB shop.
They found the CD player was cycling even when the stereo was off and the key was off! The player is on a seperate circuit with an in-line fuse located in the trunk.
The car is fine now, but I still haven't figured out what to do with the CD changer.
Thought you'd like to here how the mystery was solved.
2003 AMG S55