I'm not exactly an electronics veteran here. But I think I've heard that vehicle power is usually pretty dirty...
I'm not sure how sensitive the pi is to that kind of thing but you may be interested in getting the step down converter anyway.
Also, there's this neat WiFi-USB stick you can get. No idea how it works, but if you're trying to avoid plugging the pi into the aux input id say that might be an option... Maybe.
