ok....the guy above me is right...its going to be like finding a needle in a hay stack to find exactly what the problem is...
i had the same problems, i would get sound out of 2 speakers most of the time, and once in a blue moon, i would get sound out of all four

! it turned out that when i installed my new deck and 4 new speakers, i could hear sound.
1. check all wiring, see if somthing jiggled loose after 8 years
2. do a speaker test and replace each speaker and see if they work
3. do a deck test, maybe borrow a friends deck?
4. or just get all new gear! (my favorite)