25 Mazda3 Sedan Preferred. Rear speakers not working

[DWard]

Window sticker says Preferred has "am/fm 6 speaker audio" The rear door speakers and the single rear speaker in the package shelf do not function. My Mazda dealer service writer tells me this model has no rear speakers. I asked if they could be installed. they said no and recommended I go to an aftermarket stereo installer. Has anyone had this problem and found a proper Mazda solution? The dealership has been NO help