W210, W211, W163
Run power to the audio gateway fuse via the cigarette lighter? Genius. I have sporadic episodes of battery draining, every two or three months. Dealer has check the codes. Nothing useful. I think I may have something something intermittent in the audio gateway. I'm going to try this fix for a few months and see if the mysterious battery draining abates.
You can use the cigarette power to directly power the audio gateway but you are overloading the cigarette circuit if you do it that way.
Using the cigarette circuit to simply power a relay, which in turns powers up existing audio gateway circuit is a safer method.
I will try to do a simple sketch on how to hook up a relay. Relays are just a few box, and that way you will have a perfect solution to your problem.