confused about E320 wagon audio
 

Hello!
I've been searching the archive looking for answers but remain confused. The sound on our '94 E320 wagon has degraded into little more than a low static. From what I can tell I have a stock Becker head with a small amp in the back. There are two dash speakers, two subs in the front doors and two speakers in the rear doors.
First, can anybody tell me how I might explore to see if my problem is simply a shorted-out wire, loose connection or some other fixable solution?
Second, if the diagnosis turns out to be a bad tuner, amp, or speaker(s) can I swap out that item(s) without overhauling the whole system?
Obviously, I'm looking to save money here. Any comments will be much appreciated.