Rear Audio

About 8 months ago the rear speakers of my 2003 envoy xl stopped operating. I tried adjusting the fader control, no luck. The earphone jacks do work though. Is there a fuse for an amp dedicated to the rear seats, seams like the radio/cd system doesn't have the power it used to have, I have to turn the volume up farther now (yes, I did get my hearing checked). Thanks.