Definitly need more info to diagnose. Is it a mono or multi channel amp?
I somewhat agree with this. I can't think of any other way either channel works but both channels don't work except for a short in the signal. However, I enjoy problem solving and I loathe running wire, so I suggest some basic troubleshooting first.
Start with an ohmmeter. Each rca connection has two conductors: let's call them inners and outers.
Outers: You should have continuity between the two outers. You should have continuity between each outer and ground.
Inners: You should not have continuity between the two inners. You should not have continuity between either inner and ground.
Disconnect the cables from the amp, and measure both the cable side and the amp side.