I am wanting to add sone subs& in my car, which is a 2010 Ford Fusion. Ive been talking to ppl and some say i need to put in a aftermarket stereo then some say since i have the premium sound i can just use what i have. I just want to be sure because if i need the aftermarket that would add at least another 150 to what im doing.. If you have advice I would appreciate it.