Adding subwoofers to my girlfriends car

[soggyc1garett3]

So, my gf has a 2010 infiniti fx35 and i wanna add some subs because the stock bose system just lacks for the both of us. We would like to keep the stock head unit before we do anything else, My problem is, for the adapter harness i need, would i use that with the rca connections and run it that way?

or should i use some external speaker wire to gain a signal from somewhere else?

I feel like i’m making this more complicated than it should be, but again this is my first time being actually able to a small upgrade Thank you in advance with any information you guys have