The mp3 may not have that function with the alarm, altough it would make sense if it did. What you need to do is take it back to the dealer and have them use their DRB diagnostic tool to set that option in the preferces section of the BCM. Sometimes the BCM will trip and set that option to "no". Used to see it all the time at the dealership I worked for. Should take them less than 2 mins to do it, so you could wait I would assume. Thats about the only way I know of to turn the horn on.
