if it isn't covered i would just try and fix it myself, anyone spill soda or anything on it? a qtip with some water and dawn around that button and working it a few times might free it up. if it didn't i would remove the radio, take the bitch apart and figure out what was wrong.
my cell phone buttons quit every time i get it wet, i pull it apart, underneath the rubber keypad are a grid of metal contacts on a circuit board that the buttons touch when you hit them to complete a circuit, i scrape off the corrosion with a screwdriver and i am back in business. only do this if you are out of warranty and don't feel like getting dicked by the dealer.