Well, if you want to make your stereo go boom, you will need more then a new speaker or 2.
the 460watts is split between 4 channels, maybe even 6 I'm not sure.
Also that 460 is likely the peak power output of the stereo amp, rms is usually around half that number, which is what your stereo normally puts out, an advertising ploy.
What you need to do to hook up a subwoofer without changing the head unit (stereo) is your choice of speaker(s), an amplifier that matches the impedance of your sub's, and a thing called a "Line level convertor" This will allow you to splice into the rear channels of your stereo system, and plug RCA cables into it to provide your amp with a signal.
Now, if you just hook up a subwoofer to your factory system, you will likely be very dissapointed. Amplifiers can be designed to play a certain kind of sound, midrange or bass. If you want to add "thump" to your car, you need a amp.
Now, the speakers you have in your car now are not actually subwoofers. I could be wrong, but I don't think Ford has any subwoofers from the factory in their cars.
check out
www.onlinecarstereo.com to review a lot of products, then perhaps search for those products on ebay to find a good price.