Aux problem on car.?
Try plugging in something into the aux jack. Just a pair of headphones and then remove them? It sounds like it could possibly be the internal auxiliary jack shorting out so it thinks that there's something plugged into the aux jack. So potentially inserting a aux jack cable or 3.5 mm connector and removing it may help reset it or stop the plug from shorting giving your deck a false positive that there's something plugged in. Otherwise you may need to dismantle the deck and look at the internal aux jack and make sure the jack is not shorting out inside.
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Try a different 3.5mm wire. If it still doesn’t work, replace it.
Spray electrical contact cleaner in the jack and work the plug in and out a few times. If that doesn't correct it, you probably need to replace the jack.
A software issue