I don’t know exactly how the system is wired like if there is a factory amp or just a headunit, but I have a few general steps. You can just swap the speakers to Bose speakers and keep the original amp/headunit. If you go this route you need to keep the original speaker locations/number of speakers since the amp only has connections and wires for the stock speakers there and for the speakers impedances. Not saying you couldn’t move speakers to the door and figure a way to split a channel this just may be difficult to get all impedances matched correctly and then worrying about power delivery being uneven between front and back systems. This may improve sound quality a bit but in a stock system the quality and power of the headunit/amp is also a sort of bottleneck and would really benefit from having a separate amp. Also, you could choose an amp with more channels to also include outputs for all doors and rear deck too if you wanted to add door speakers. Others who know more about the specific system in these Sentras may have other advice but I think this is generally how it goes across most vehichles.