The radio provides a four-channel output to the amp. If the radio's rear-channels amp circuits are broken or the radio's internal fader circuits are broken, then you would get no rear channel outputs from the radio. That is my guess for the most likely cause.

If you were ambitious, you could swap the front and rear signal input wires at the amp plug and see if you suddently get rear channel audio (and no front channel audio). If so, then you would confirm that the radio's outputs are the problem.

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