We're happy to lend a hand with your iPhone and the Bluetooth connection.
Unpairing and pairing back together is a good step to try. Here are some more steps that you can try.
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. Make sure that your 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.
These are found in How to connect your iPhone, iPad, or iPod touch to your car - Apple Support along with additional information you can try.
Should this not resolve your issue with Bluetooth, you will want to use the link above for contacting Apple Support.
Good luck!