Not connecting via usb since latest update
What protocol does the connection to your car radio use? I can't exactly speak for the GS3 but this is for stock Android so it may be similar. Go into Storage under Settings and hit the menu button (or the menu overflow) then select USB connection (or similar), make sure MTP (you probably need this one) and any other protocols you need are checked. There may also be a Mount USB option present if that the former does not work.