Can't install new receiver...
 

I just spoke to some audio guys, and they said that I may not be able to install just a new head unit in my '99 e430. The book they pulled out said that if fiber optics are installed, I have to rewire the whole thing (buy a new amp as well). Frankly, they confused me... How do I know if I have fiber optics? Is it only if I have a car phone installed?

no wayy
 

they are full of it! you dont need to rewire the entire car. All you need is an adapter that would go from your head unit to the bose amp. It is obvious that they have no skills. Don't let them rewire your car, you system will sound like crap.
Everything is possible as long as the person knows what they are doing.

BS - These people want to steal your money
 

The reciever itself is not Fiber Optic. Its an Alpine with FO interface for the CD changer. You can use any receiver as long as it fits, and you can correctly hookup to the Bose amplifier built into the car to support the active speakers. That part is usually provisioned by adding a translator box (turns your recievers HI output into LOW level input and provides WAKE msg for the amp.) These interfaces are very common and inexpensive.

To solve the Changer problem, you might want to run cable that comes from the manufacturer from the center console to your trunk (No big deal) for any decent installer.

Looks like these people do not know what they are doing, and more so, probably they DO and wanna take you for a ride by stealing more money from you.

Last, but not the least, IMO you can get any reciever, but alpines do very well in MB. Also you might be able to find one with FO changer capability. I do not know if one exists, but they might since they make these for MB any way.