SatNav makes sense but with the car charger so close I'm not sure you would need it. I would buy one though if it was the same price, the wire has a habit of tangling its way around the gear lever on mine.
As for mobiles and MP3 players, well they spend most of their time in my pocket, and when they are out they are face up, so I can see when people call or text, so that wouldn't work in my books.
Their are some good videos on Youtube demonstrating how to take one of those cheap garden lights that you mentioned apart, and attaching it to a phone charger. I might have a go at that one day, it would be handy for camping.