Why did my whole audio system stop playing except front speakers?

I recently purchased the Kenwood KMM-BT325 bluetooth media player for my car audio system. When I first got it installed I just ran it on the internal speakers that were already in my car. Later on I purchased a Memphis Audio Amp with 600 watt PEAK and a good sub. After I got it installed I was so happy with everything. It sounded amazing and it was not to much so I did not attract unnecessary attention. Well about 2 months later when I turned my car on my amp would not kick on I would have to turn it up really loud to get it to start hitting then turn it down. Then I noticed the actual radio would turn off and back on sometimes when I push the next song button. Then everything went out except the radio and the 2 front speakers. So i was looking at the wiring and everything cuz i thought it might be my amp but when i looked at it while my car was on and the amp's light was on so it is for sure getting power but the sound from the sub and back speakers is not playing at all.