Nah, you COULD use your existing deck to install a sub. Problem is, you gotta give up the rear speaker output (if there are no RCA prouts on the deck). Also, you've got to get an amp that accpets hi-input(this would be your speaker leads) or a speaker to RCA converter, and then to the amp.
If you're wondering if a subwoofer can be powered by the deck, chances are, No. You STILL need to get an amplifier.
CHoosing a subwoofer for you might be fun, and can also be an ordeal. You've got to know what you expect by adding a subwoofer into your system.
SImple questions like what music do you usually listen to, do you want pounding bass, or perhaps more civil quality bass (most people think they want this, but in fact want SPL), what kind of budget do you have to work with, what kind of amplifiers, how much space in the boot you are willing to sacrifice.
//content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif hope that helps a bit.