96 Suburban right, and rear spkrs stopped working

Hello All,
My 96 'burb's radio lost the right passenger, and two rear stock speakers, or so I thought.
I figured the odds of 3 speakers going at the same time was pretty high, so I repalced the radio (wanted a newer one anyway), but no sound from the 3 spkrs. The tech at circuit city where radio was installed said the spkrs were working it 's a wiring problem. Is there a site where i can get wiring diagrams of the suburban, or has anyone heard of anything like this before?

Thanks for any ideas.

kjohn