RiNkY,

build a sealed box for a nice 10" sub. I would recommend a JL 10w3v2. Use 3/4 MDF + wood glue + clamps....piece of cake. To power it, put a JL250/1 under the passenger seat. It should fit w/o too much trouble and then you only have to have your sub in the back. When you want to pull it for hauling stuff, you can and you'll just have to disconnect the speaker wires. It's not optimal, but you can just run the speaker off the head unit if all you want is some added bass.