Q: I have a problem between the audio system of my car and the bluetooth conection to my new iPhone

Try un pairing then re pairing the iPhone.

Instructions here > Get help connecting your iPhone, iPad, or iPod touch with your car stereo - Apple Support

   IOS user Melipattz, 1) did you set up your 6s plus from the cloud or iTunes back-up from your previous 5s?? The peripheral device could be starting to link the same iPhone 5s's bluetooth signal, but the blue-tooth address, thinking it is the same device because of the device's discover-able "name" might not have been changed yet (By default if an icloud backup was used to initially set-up all of your new devices data, contacts, messages ect...it will have back-up's namesake, but a different Bluetooth address will not allow it to complete desired pairing of the 6s plus ((the default name possibly?!)) Sound familiar? If so...1) Go to settings, general, "about device" top line; re-name your device, restart of your device & try once again to connect via Bluetooth