Hey there Christel2012,
Thanks for reaching out.
To clarify, was the old iPhone ever connected to the car, or was the iPhone 13 the only device connected?
If possible, try removing the iPhone X connection from your car stereo settings directly so this will permanently remove the connection. This will also prevent it from connecting in the future. You'll want to consult with the dealer or instruction manual to remove it from your stereo.
In the instance it connects to your car again, would it be possible to access your iPhone X so you can check the Bluetooth settings there in order to remove the car's connection? If so, you'll find those steps here:
Go to Settings
> Bluetooth, tap the information button
next to the name of the device, then tap Forget This Device.
If you don’t see the Devices list, make sure Bluetooth is turned on.
If you have AirPods and you tap Forget This Device, they’re automatically removed from other devices where you’re signed in with the same Apple ID.
Set up and use Bluetooth accessories on iPhone - Apple Support
Take care!