it's very easy to wire a amp+sub to ur existing sounds system.
might not be the best way to do it but it's the easiest for sure. just take one of the rear speaker wires (not the tweeter) and splice into it and feed that wire to the new amp + connect new amp to the battery + connect sub to the amp and ur done.
mine has been running like that for 6 years now all on stock deck and stock amp still being there. i dont wanna upgrade the deck cuz it's just gonna ruin the looks of the front, and i'm not too much into the whole audio stuff. as long as the seats vibrate i'm happy.