Hello I got myself a 2014 mkz with the HD radio system with 11 speakers. I also notice that the rear speakers were very low, so I started to check the settings and notice these three buttons on the screen, Front, All Passenger, and rear. It was setup in All Passenger, so I switch it to front and all the volume stayed in the front. Then I switch it to rear and then the sound came out equally thru all the speakers and it sounds great.