I've been combing the forum for relevant posts that share similar issues as myself.

I too have been having a power drain in my 1989 300E. My drain is coming from the "Check Engine" light that stays on while the car is completely shut off. Strange thing is the "Check Engine" light goes out when I turn the car on and stays off until I shut the car off again.

Here is the link to my thread. Hopefully some information can be gained from somewhere to solve these issues.

Check Engine Light On While Engine Off II