Aux port in car stereo not working properly

[Boots96]

I'm plugging my iphone into the aux port using a 3.5mm to lightning cable. Recently when I try to play music this way the songs vocals are very quiet, but the instruments sound fine. I've swapped cords, but that didn't help. One thing I noticed is that if I leave the cord plugged in to the stereo overnight, then the issue persists. If I unplug the cord when I'm done with it, then plug it back in the next day after I've started the car, it works normally.

Any ideas?

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