Hello and welcome to you. Sorry I can't help, but I feel sure someone will be along to answer your question shortly.
I'm still trying to sort out this new fangled 8track stereo.
Where is the USB exactly? Likely, unless you have opted for iPod Media or Media Interface (or is included) it is either a phone charging point or it is a means of playing music from a USB device. The latter is unlikely, I would think (but willing to be convinced otherwise), as that would require IM or MI.
Have you tried bluetoothing the iPhone to the vehicle yet? I don't know what version of Audio is in the new Sprinter but some versions give, when bluetooth connected, phone, contacts, music streaming and control on the steering wheel. I guess the clue will be whether or not you have buttons on the steering wheel.