My daughter's 2004 has the moons and tunes option and I believe the factory amp has failed. There is no sound on the driver's side and this problem persisted even after replacing the head unit.
Does anyone know if the factory amp was used in other cars?
I would try to pull one from a junk yard, but Vibes are not easy to come across, so if this amp was used in other GM models that would widen the search.