It might help if you state which phone model do you have and which version of Android it's running.
Different phones have different versions of Bluetooth, and that might reveal something.
But outside of that, go to your car's manufacturer's web site and see if there's a help forum or FAQ page for more direct, relevant help. Someone with the same make or even the same car model as yours might be able to provide actual, hands-on info on pairing up a Bluetooth device. Or at least tell us what car you have, in the event someone has the same or similar.