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I'm afraid this is most likely an internal issue in the radio.
The wiring runs directly from the radio to the auxiliary jack with no other modules or anything in between that could affect operation.
The one external thing that could possibly cause this is if the wiring from the radio to the jack was shorted. It's possible that shorted wiring could be causing the radio to not recognize that it has the auxiliary function but it's not that likely.
What I would first try is a battery disconnect, which will sometimes fix issues like this. If that doesn't help then I would next make sure the jack's wiring is ok, and if it is then the radio will need to be replaced.