I have the same problem since I update my iOS to 11.0.3. Before the update, the connection with car media never have a fail, and it required restarting device with the new IOS.
I checked all other functions and nothing goes wrong other than that.
I have also followed the procedure you recommended, but it only work if I restart my iPhone with the car media on, once the car is off and restarted, the connection failed again.
I believe Apple need to do something with the new iOS.