That sounds like either a bad radio module or a bad connection between the radio module and DSP amp.

The radio feeds just left and right channels to the DSP amp. So, if you are missing one whole side, it is an output thing from the radio not reaching the amp. I would disconnect the plugs from both. Maybe it is just a loose connection. You might also try connecting a voltmeter to the radio wires to confirm that you are getting an output signal at various points. Or, if you do not have a voltmeter, you could connect a cheap speaker to the wires to confirm output.

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