An FM/AM antenna is a wireless receiver. Not a transceiver (though CB is and is still common in long-haul trucking), but still, it's always been possible for someone to hijack the channel and send an unwanted signal into your speakers. That's how pirate radio worked for a century. If all an attacker can do is send sound into your cabin, that isn't a meaningful vulnerability. They could do that with a megaphone.

If car makers are actually hooking the bluetooth chip up to anything except the sound system, that's the problem. Although, assuming a class 2 device, it only has a 10 meter range. If an attacker is that close and wants to shut down your car, they can use a nail strip.

The real problem here is hooking up WWAN to control software.