I assume you have the Bosch 80A alternator. My 300TE had this alternator and I never had a problem with it keeping up with the discharge rate but many people on this site have upgraded their alternators to a 143A alternator to cure the situation you have where the discharge rate exceeds the output of the alternator. This is especially true in hot weather where the auxiliary fan runs on high speed, headlights on and aftermarket stereo pumping. Here is a link to one conversion.
http://www.peachparts.com/shopforum/car-audio-multimedia/53236-installed-143a-alternator-my-w124-86-95e-class-photos.html
If you haven't had a problem with this in the past simply replace your alternator with a quality rebuilt stock alternator and enjoy your MB. Good luck, Mark