I recently came into this same situation. I ended up getting a TMA 12inch sub in a box and a 100watt amp. Both the stock and regular sub are hooked up and i gotta say this is all you need if your just looking for a nice setup for some extra bass..i paid $300 for wires sub, box, amp, and installation The aftermarket sub is behind third row pointing toward the passenger side and the stock sub is pointing toward the drivers side. I haven't noticed it hard to tune at all. And it really sounds amazing no rattling either!!! I will take a picture soon
The TMA sub that I got is made by JL audio and it really is quite good ive been through alot of subs and this one was a pretty good bang for ya buck lol