I have been checking with several good aftermarket shops and you can change the head unit on the 99 Mercedes. In fact if you got the Bose sytem, here's the best approach. Pull the head unit and push the connector to the rest of the Bose system out of the way. Disconnect power to the Bose amp. Replace the front door speakers with a mid and put a tweeter in the existing tweeter enclosure in the door pillar. Put a good 6.5 mid in the rear deck for fill. Ignore the now nonfunctional rear door speakers. Put in a decent head unit to power the new front and rear speakers. Now you have a sound system that should have come in the car. Nothing exotic - just good clean sound.