Could be a bad output on the head unit? Is it factory? Genital Motors head units are notoriously pieces of crap. I replace them on a daily basis. CD players don't work, outputs don't work, displays don't work, blown speakers, bad amps, etc. I have rarely however, worked on a GM car with bad stereo wiring. It's usually a bad component. Just my opinion. Hell, the display on my GP head unit doesn't work, and 2 of the speakers are shot, but they still work. The surround is shot around the cones. But on your car, my money is on a bad component, unless someone has messed with the wiring or the car has been in a wreck on that side to compromise the wiring.
