they are probably right about loosing the voice stuff coming from the navi. if its similar to GM vehicles with OnStar, which runs all you voice stuff through the factory radio, you can't put a aftermarket head unit and have it integrated with your navi, or in the case of OnStar ppl their OnStar system. like e3NiNe said, the factory head units use a port on the back of the radio for accessories to be plugged into.