What cables in the back will that be?
The amp is above the headlining and above the antenna in the rear off side glass, but no good in just looking at it, ok if you have the skills in testing amps other wise its testing by replacement, which also goes for the head unit which may have a fault. I suppose you could have the head unit out to test the coax wiring continuity from the head unit and the supply to the amp is ok.