Ok, Im looking to install the front and rear speakers. Does changing just the speakers will make any different or I would need a new amp? If I was to install a 4-ch amp to replace the stock amp, would I lose the NAVI voice?

 

I assume since you have navi you also have the Bose system. If you only replace the speakers, stick with some rated at 2 ohms. If you keep the stock head unit and wiring and simply put another amp"in place of" the Bose amp, you should be able to keep the navi voice.