Hello nathanael134,
Welcome to Apple Support Communities!
We want to help get your car playing music like you expect from your iPhone.
Try these steps to select your car as the audio output when connected:
"Switch manually
If necessary, follow these steps to choose a different device to play audio or to switch the audio to different headphones or speakers.
On your iPhone, iPad, or iPod touch
- Open Control Center on your iPhone, iPad, or iPod touch.
- Touch and hold the audio card in the upper-right corner of Control Center to control audio.
- Tap
, then choose the device that you want from the list."
This Apple Support page has the full steps even though the title doesn't match the issue: Switch your AirPods to another device - Apple Support
If the issue continues, do you have any other paired devices that might override the connection?
Does the same thing happen after restarting your iPhone and trying again?
We hope this helps to isolate and resolve the issue so you can enjoy your music in the car.
Cheers!