In reply to:
On a rare occasionally the right channel will play as normal but then slowly fade in volume,indicating(I think)a faulty component and not a loose connection.
I wouldn't say that. I had a problem like that with the right channel that only existed in the harness, but anyway, it would fade in and out and it definitely was a loose connection and not a faulty component. Your problem sounds like a loose connection to me. As the others have said, check for bad solder joints.
No, there are no schematics for the general public.
Stu