Why no rear speaker audio

Are you on Bluetooth? If so check to make sure that the rear speakers are enabled in the Bluetooth settings. Kenwood head units are known for this issue so if you have a Kenwood definitely check this out. If you go into settings, then to detailed settings you will see an option for something like Bluetooth HF audio. Go in there and you should see that only the front speakers are enabled. This is the default on their head units for the people who just use Bluetooth to talk on the phone and not stream their music. If it isn't kenwood it may have a similar setting. Also you could try checking the "advanced" or "pro" setup options to make sure the speakers are enabled there too. Hopefully this helps!