Hello, Bromthorn. Thank you for using the Apple Support Communities. We understand your situation and how you would like to prevent music from playing automatically when you connect your iPhone to your car’s audio system. We’re happy to research any available options for this.
To get started, we would suggest checking your car’s stereo settings to see if autoplay is a feature that is supported and enabled at this time. If needed, you may want to disable this feature from your car’s audio settings. Please consult your owner’s manual for help with finding these settings for your stereo.
If you continue to notice the same issue after this, we can help you more. To continue, can you tell us if this happens when you connect your iPhone with other Bluetooth audio accessories as well?
You can also prevent cellular data access for apps that are playing content automatically. This would prevent the app from using cellular data to stream content. The following Apple Support article explains how you can manage the cellular data settings that your iPhone apps use:
View or change cellular data settings on iPhone - Apple Support
We hope that this answer provides you with the answer that you need. Please let us know if you have any other questions.
Cheers!