I think Peter gets it right most of the time, but I suppose that there are other glitches in stuff like the speedometer, the wiring and the mechanicals that he doesn't get to check and therefore can't fix if they are broken.
I am sorry you are having trouble and hope you get it working well soon. Perhaps I am lucky. My 300TD came with no amp at all and I had to send him one from a junker that luckily found defunct for $20.
Semibodacious Transmogrifications a Specialty
1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)
"Time flies like and arrow, yet fruit flies like a banana"