Amp works fine, no sound thorughout vehicle...

I have a Kenwood headunit and amplifier, 1000W. For a little over three years I've had no problems with it. Never pushing anything too hard. Connected to the amp are four 4x6 rockford fosgate door speakers, and two 12" pioneer shallow mounts. A few months ago the bass began to go in and out periodically. when it went out all I had to do was turn the power on and off and it would come back. One day I turned the vehicle on and there was absolutely no sound coming out of the door speakers/subs. Nothing is wrong with the head unit, it has been tested. Anyone have any idea??