Some reasons (I am assuming you are talking about hardwired modulators):
1) to use in most Mercedes, you need TWO adapters... so your antenna cable will have four connections instead of one. That creates signal loss.
2) the modulator must be powered off to get "normal" FM stations. In most installs this involves an additional action on the user's part to disable the modulator. Add this to the additional action of selecting the proper frequency to listen to the new device.
3) in some cases the modulator cannot be set to a "clear" station. This isn't too much of a problem because most modulators will block the incoming antenna signal.
4) finally, some Mercedes cars have antenna boosters powered FROM THE ANTENNA WIRE (not from the antenan power lead). The adapters and modulator will BLOCK this power. This can be fixed by purchasing one of the Metra (or whoever's) power adapters that takes a 12v input and feeds it on the antenna lead.
BenzWorld.org forum is one of the largest Mercedes-Benz owner websites offering the most comprehensive collection of Mercedes-Benz information anywhere in the world. The site includes MB Forums, News, Galleries, Publications, Classifieds, Events and much more!