Details always help. Which iPhone, which iOS, which version of software are you on in the car, etc. Have you tried another iPhone? Like an old one, a spare, your spouse's, etc. That'll help you isolate the issue, is it the phone or the car, etc. Try rebooting the car, try rebooting the phone, try repairing, etc. In general BT issues are rare, most people try to setup their phone for music and are not setting up their phone as a key, that sort of thing.