Go into your car's infotainment system and delete your phone from the list of connected devices. Next, open the Bluetooth settings on your phone, and "forget" the connection to your car. Immediately after, power off your phone.
Once you have completed these steps, turn the ignition on in your car, and browse for Bluetooth devices while your phone is off. As your infotainment searches for devices to connect to, power your phone on. If your infotainment fails to "find" your phone in the alloted search time, retry. Once your devices "see" each other, pair them (there may be a 6-digit pairing code displayed on both devices), if the pass codes match. Finally, check that you are able to switch on audio pairing once more.