USB data transfer with a separate power source
It should work, but you won't get more than 500mA. When a USB device, such as your iPhone, is connected to a data port, it asks the port how much current it can supply before pulling more than 100mA. Since the car doesn't know there is a 4.2A supply available, it will report '500mA'. The iPhone will then draw up to this limit from the port.