Q: Unsuccessful dealership repairs for stereo/navigation system

It must be 30 times that I have been to the Mercedes-Benz dealership. They either can't fix the problem or else they are swapping car parts around and thus I have a continued problem with my stereo/navigation system. About once or twice a month, sound doesn't come out of the left-side speakers. It is always the left side only. One thing that does solve the problem is to restart my car and let it sit for a few minutes (or sometimes hours) after which the sound will return. But what a pain and loss of time.

Twice the dealership has changed the headunit and the amplifies - but the problem still persists and last month it broke again. The mechanic replaced the headunit once again, but I found out that he installed a "Remanufactured" unit!! I didn't know that Mercedes-Benz uses refurbished parts. After getting my car, the navigation system broke down while driving home. I called the Mercedes-Benz representative and was told they don't make new parts for headunits. They haven't responded further, for three weeks, regarding my inquiries that I want a 'new' unit. I'm thinking the stereo problem is not with the head unit because that was replaced before and it never solved a thing. Can you please give me advice on what to do in order to get this problem repaired and fixed? Thank you.