I have a question about getting a new car stereo for my 2006 wrx (want aux input for my mp3 player). I have the factory sub/amp and cd changer already installed. When I look at stereos on crutchfield, it tells me that I will lose my factory sub and/or amp if I replace the head unit. I wanted to see if this was true or not? How can I get a new stereo and keep the factory sub/amp?