Check your Car Audio System
I have two cars one with Sony DSX A410BT and the second one with Sony MEX BT3153U
My iPhone 6 iOS 12.3.1 when connects to these audio system
As soon as the ignition is put ON in the car with the audio system, Sony DSX A410BT connects automatically to Bluetooth and then automatically opens iTunes and automatically starts to play from where it last left off.
All Automatic: Bluetooth connection, Opening iTunes, Song selection, Play
But, But - Same iPhone
As soon as the ignition is put ON in the car with the audio system, Sony MEX BT3153U does nothing automatically
Manual Connection: Connect Manually to Bluetooth, Manually Open iTunes, Manually Select Songs, Manually Play
How to use CarPlay with your iPhone - Apple Support
https://support.apple.com/en-in/HT205634