2010 ultra. Factory radio. When I fade to rear speakers, no sound comes out unless the volume is all the way up, then I can barely hear the sound. I toggled the speaker switch on dash, no effect.
Bad speakers, wiring, bad amp in the radio, hard to say without a bit more trouble shooting.
I would try a different speaker, Perhaps pull the outer fairing and unplug, and plug back in all radio connectors.