rear speakers don't work at all

1998 S500 Bose stereo. All front speakers work fine, balance works fine. No sound out of any rear speakers. Amp looks good, no mods done, all factory. Owned since new. Fuse at amp is fine, just no sound in rear. The fader seems to work, but when I send all the sound to the rear, I do still have about 1/4 volume in the front speakers. Any advice would be appreciated. From all I have read on these forums, it doesn't appear to be the amp. Maybe the radio itself?