I pulled this from the site https://support.apple.com/en-us/HT205634:
- Make sure that Siri is turned on.
- Restart your iPhone and your car. Depending on your car, you might see the CarPlay Home screen when you turn it on. If you don't, look for the CarPlay logo on your car’s display.
- If possible, connect your iPhone to a different USB port in your car.
- Try a different Lightning to USB cable. Make sure that the cable is certified.
- Update to the latest version of iOS.
- Check your car’s manual for help. You might need to install firmware updates for your stereo.