iphone 8 plus bluetooth won't play music

Good afternoon Rob16121612,

Welcome to the Apple Support Communities!

I understand you are not able to hear music playing from your iPhone 8 through your car stereo via Bluetooth. iOS 13.3.1 was released today, January 28. I would suggest unpairing your iPhone from your vehicle. Then update the iOS and see if there are any updates for your vehicle as well. Restart the iPhone and the vehicle and pair the iPhone again.

Update your iPhone, iPad, or iPod touch

Connect Bluetooth devices to iPhone - iPhone User Guide

Play music, podcasts, and audiobooks in your car

Also, check out the troubleshooting steps in the following article: How to connect your iPhone, iPad, or iPod touch to your car - Apple Support

1. "Restart your device.
2. Go to Settings > Bluetooth, and turn off Bluetooth. Wait for about 5 seconds, then turn Bluetooth back on.
3. Check the manual that came with your car for more information on how to pair with a Bluetooth device. Most cars require a phone setup on the car display. 
4. If you've connected multiple phones to your car stereo, try renaming your device: Go to Settings > General > About > Name, and type a new name. Then try connecting again.
5. Update your device.
6. Make sure that your CarPlay stereo is using the latest firmware from the car manufacturer. Check your car's manual for more information.

If you still can't connect, contact Apple Support."

Take care.