2004 Sierra - Bose stereo - rear speakers quit!

I have a 2004 Sierra crew cab with a factory Bose in-dash cd changer and DVD entertainment system. The front speakers sound awesome, but the rear speakers are not working. Does anybody know what I should check first or if this sounds like a common problem?
Thanks

Did you just buy this vehicle or have the speakers suddenly stopped working?

Check the radio settings first, kids and people fool with them and it may be something that simple. Sounds strange for both rears to go out so it would suggest a radio setting not right. Thee are normally 2 settings, fade (front to back) and Balance (Right/Left).