Try this first on a computer. When you connect your phone through USB there should be a legend on the notification bar saying that the phone is either just charging or if it's ready for file transfer (THIS is the mode you want!). Now, depending on your car system, if it doesn't support MTP and it needs generic USB Storage profiles, you won't be able to use your phone with that car system, unfortunately. Some people have to Enable USB Debugging in their phones (it should be in your settings, it's not an app, and you may have to enable Developer Mode on your phone to see that menu) in order to get their computers to recognize them. You might want to try that and see if it helps with the car.
The 'official' version is that Generic Mass Storage was removed from the OS for security reasons (and a valid point, to a certain extent). As to why the Audio through USB driver wasn't included, you'd need to ask the car system manufacturer... the ability to do it IS in your phone, it's the accessories that don't implement the protocol.