From what the manual states, the reverse sensor is disabled when you hook up the trailer lights. I assume that its for the overseas variations and that its also factory installed. If you could find a wiring diagram for the factory wiring you may be able to wire yours in a way that would disable it.
If it works off of a separate speaker (I wouldn't know mine hasn't worked at all and its going back to the dealer soon) you may be able to put a switch in front of it.