If the car has the Blue & Me system then you won't be able to easily get AUX input to work. You'll find that with the ignition off, you should be able to cycle to AUX, but as soon as the engine starts and Blue & Me kicks in, it'll stop.
Some users have worked around this by loading a silent, several hour long MP3 file and inserting it into the USB port. Playing this through the MP3 player, with AUX running at the same time has worked, but not for me.
If you just search for this on Google you'll find lots of articles from users who have had varying levels of success doing this.
