Ok, I'm not familiar with the model of head unit you refer to. Maybe it's simply not possible to connect this way.
Is it at all possible to contact the manufacturer and get there advice.
As I said, I've never come across a car head unit that connects an iPod via a usb connection. My Kenwood unit has a usb connection and will not recognize an iPod connected in this manner (using the iPod cable). The Kenwood needs the specific iPod multi pin connector.
Also, simply connecting via usb will probably not supply a charge to the iPod from the head unit.