Have you checked with Hyundai? It would be a disaster if they are not providing rear speakers. But then again, you're saying there is music being played from the rear speaker locations, only very faint. But normally, this menu would have the fader in it too so this is really interesting. Even if there aint no speakers, the option of fader should be there by the manufacturer since in most case, people do install rear speakers if not provided by companies like Indus and suzuki
