You have many options. The easiest thing to do is simply leave the factory head unit intact, and install an amplifier with "high level" inputs. Basically, you'll tap into your rear door speakers or any speakers in the car and run the speaker wires into the high level input on the amplifier. You don't need to disable whatever speakers you tap into, but run another set of wires from the ones feeding the speakers. Alot of newer amps have "high level current sensing" which means they'll power up once they detect audio coming into the high level inpuyt. If you go this route, you'll probably geta little bit better sound if you use an external high level converter - a small device that will actually connect to the speaker wires and have RCA plugs on the other side to connect to an amp. You can find a decent one at an electronics retailer for $20-$40. The second alternative would be to buy an aftermarket Head Unit and then use an integration adaptor to continue utilizing the steering wheel mounted controls. My V came without the convience package, so I don't know what controls are on the steering wheel, but you may very well lose a few of the functions if you go that route. Someone else can confirm if they did it or not, but you would probably lose the bluetooth feature if you did it that way, and to be honest, I haven't even checked if there is an integration harness available for the V. Plus, if there is such a harness available, you would be limited on the brands and types of head units you could use it with. The quickest and easiest route would be to just do what EV recommended, but it depends on what kind of setup you're looking for. Infinity used to make a product called the Basslink, but I'm not sure if it exists anymore. It is much like the product from Kenwood that he recommended - a completely self contained amp, subwoofer and enclosure combo that simply requires power and a signal from your speakers. A decent stereo shop can help you out with all of this. Or if you prefer to do it yourself, a good resource for learning would be
http://www.crutchfield.com