alright here the deal i install a custom subwoofer box in my car yesterday and and after hooking it all up my sub's will not play and my front door speaker and tweeters will not play ether only speakers that work are my speakers in the back which are hook up to my cd player only. so i look around the back check my wire to see if maybe a wire came lose and everything seem fine and i check both my amp's and they both show power as well. well after i got up today i went out to my car to do some more troubleshooting and right as i turn my cd player on my back speakers made a popping sound and each time i change from cd player to usb flash drive or radio it will make the popping sound again.
Sand the ground to chassis connection of your amp in the back and the grounding of your HU and recheck your RCAs (better get new ones). Also you need to check the voltage on the amp's power input to be 13-14V when the car is on. To do those tests you will need a $10 multimeter found at Walmart.. best investment ever for car audio installation.
what made me look was i was getting a reading of 6.8v coming out each amp and i was getting a reading of 14.2v up front under the hood so i knew it was either a bad power wire or a bad ground.