the radio will shut off for 3 seconds then turn back on. repeat. as long as the car is running.
Take it to a shop that specializes in car audio. It could just be a loose connection but I doubt it. You’ll probably need a new radio.
Or, it is conceivable that a failing alternator–with wildly varying output–could do the same thing. In fact, that happened to me many years ago, for a few days before the alternator totally died and stranded me.
I had the radio in my 2013 Equinox reflashed twice under warranty due to the radio turning itself on randomly or not shutting off after exiting the car. Might be worth a call to the dealer or Hyundai to see if it’s a known issue.