Here is the hi.lo converter that you will need, only $15 shipped.
I used it for 3 months and so far, very happy with it. Installation was easy because everything you need are in the trunk.
By the way, anyone know any method to have more sound coming in? I have MTX 421D with 420w RMS connect to 2 10" Kicker CVC. It's very loud when the trunk is opened but when it closes, inside I can feel it, but don't hear much. I don't have the seat fold down option.
Another tip for sub DUYer, if you don't have a port sub box, make sure you add poly-fill. My subs hit a lot harder after 20 oz of poly fill at walmart for 2 bucks.