2008 Chevrolet Silverado Radio speakers

All the things you mentioned are built into the Body Computer which GM cleverly built into the radio. Most likely you have a remote amplifier that runs the dead speakers, but the working speakers don't go through that amp. The only thing you can do is check for a blown fuse. If they are okay, you're tied to the dealer for this. If the radio has to be replaced, it will have to come through the dealer and they will have to program it to your vehicle.