Fiber optic system

I have a problem with my fiber optic system in my 02' ML320. The dealer says its my amp, but I sent the amp out to a repair shop and they said it works fine. The repair shop suggested it must be in the fiber optic system.  I replaced the radio so I know it's not the radio (the dealer also confirmed the radio works fine). I'm having difficulty finding a repair shop for the fiber optic (other than an MB dealer). Perhaps I should try another MB dealer? Any suggestions? I live in Atlantic County in New Jersey but I'm willing to travel to PA if necessary. Any thoughts or suggestions? \

Bob