Hey there DMH.,
Thanks for reaching out to Apple Support Communities. I understand you’re having issues connecting your iPhone to your car stereo via USB. I’ll be happy to help with this.
It sounds like you’ve done some great troubleshooting on this so far and the following steps may resolve this:
-Try to connect using another cable or USB port.
-Check your car owner's manual to learn how to connect an Apple device.
-Check your charging port for any debris or damage.
-Make sure that your charging cable is inserted fully into the port.
-Back up and update your iPhone to iOS 13.2.2:
How to back up your iPhone, iPad, and iPod touch
Update your iPhone, iPad, or iPod touch
-Install any firmware updates for your car stereo.
You can find more info on this here:
If your iPhone, iPad, or iPod doesn't connect to or work in your car
Cheers.