Can't install new receiver...
Everything is possible as long as the person knows what they are doing.
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.



