So yesterday I go to start my wagon (having parked it just the night before), and the guages just bounce about wildly. I pull out my multimeter, and sure enough the battery's only at 9v. I pop in my spare battery and off I go. Later I park it and keep an eye on it. After about an hour the voltage has dropped from 13.5 to 12.5. The alternator's pumping out 14v when running. So I disconnect it and come back the next day.

As for today... I did the lightbulb trick. Grabbed a spare bulb, put it on the battery post, and touched the terminal to it. Lights up brightly.

Next stop: fusebox. I grab the first fuse to catch my eye, which just happens to be #2. The bulb shuts up. This covers a few of the interior lights, but the fact that the antenna is also on the list is noted because of a search I did earlier on this site. The antenna motor seems to work fine, and the motor shuts up when in the off position. However, I disconnect it anyways. Bingo (hopefully). The bulb has shut up, but I'm going to check the voltage again in the next hour.

weird problem, but i don't quite understand what your solution was.

The antenna motor was draining the battery somehow. I unplugged it.

Seems ok so far, although the battery light in the instrument cluster lit up this morning, whatever that means.

could be the voltage regulator on the alternator

http://dieselgiant.com/mercedesvoltageregulatorinstall.htm