Yes, you need an amp, an RCA line converter, an amp wiring kit, and some time to install it.
I have helped install a couple differnt systems. 2 x 10's in one and a single ten in another. The cheaper system is my favorite good price point and excellent results. It costs less than 300.00 and consists of a single 10 kicker sub and ported truck box. The box and sub are sold together for about 100.00 at Circuit City. It fit nicely on the back wheel well where the old sub was and its easy to secure in place. It really helps the sound quality of the FJ stereo. I think it is more than enough bass.
However, it you want to explode your head and disturb the peace 2 x 10's and a 300 to 400 watt. amp should be enough to get the job done.