2000 Camry speaker problem

The rear speakers for my 2000 Camry don't really work. If I set it so that only the rear speakers are on, and I turn the volume all the way up, not much sound comes out of them. What sounds does come out sounds very "transistor radio"-ish. Very tinny. Both speakers are having the problem. Are the speakers just bad or could it be something else?

Check the connections on the rear of the head unit.

See this website:
http://www.installdr.com/

I pulled the radio out and noticed that there was a 20-pin wire harness connected to it. There is also a 12-pin connector ontheback of the radio but nothing was connected to it and I didn't see any loose cables. Is this is how it's supposed to be?

See the above website.