Now you're left with installing the subwoofer cabinet. Since it'll be located in the box of your truck and generally exposed to the elements, you wouldn't be using untreated MDF. The outside surfaces of the cabinet need to be saturated with fiberglass resin to prevent the wood from absorbing water. If you haven't mixed fiberglass resins before, we've a short module as part of our online training course on our video site at www.mobiledynamics.tv/BCO1.
The final step is to secure the cabinet to the box and make a weatherproof seal between the cabinet and the box. Any leakage here will likely wind up soaking the carpeted floor of your interior. If the cargo area is wide open, you'll need to build a drip channel around the opening and install a thick, closed-cell neoprene weather-stripping gasket to keep water out.
In many cases, the idea of building an "outdoor" speaker cabinet out of fiberglass floats to the top of the heap. For any exterior project, I'm right in line with a material that has incredible resilience to the elements. However, I weigh that against the knowledge that fiberglass is best used when the structure isn't flat. In order to get some decent power handling, the walls would need to be at least as thick as the equivalent (3/4-inch) MDF to reduce bass-wasting flex.
Fiberglass gets its strength from curves, so unless you decide to make a more artistic-looking cabinet (a good idea all around), it would be better to resort to the old technology or MDF, providing all interior and exterior surfaces of the cabinet are laminated with a single layer of fiberglass and resin. If you happen to have a fiberglass tonneau cover for your box, your job just got a lot easier.
As for the Xbox, there are a couple of power supply options. One is to run a converter that will provide the +12/-12 volt rails, a couple of 5-volt outputs and a 3.3-volt output. This is the "modification" method, which is probably more efficient but harder for the enthusiast.
The easy way is to simply drop in a 12-volt to 120-volt inverter sold just about anywhere. In this way, you're stepping up the voltage, by inverting and transforming up to 120 volts, and plugging in the Xbox. Essentially, you're installing the equivalent of a household AC supply in your car.
To select the right inverter, the specification you need is the power consumption. If the Xbox consumes 140 watts, buy an inverter that will provide at least 20 percent more wattage; in this case it would be about 170 watts. I don't know the Xbox consumption off hand, since I chose the PlayStation 3 due to its Blu-Ray capability (good call, since recently HD DVD was discontinued). This is important to prevent running the inverter near its maximum. A bit of headroom is always a good thing. The price is usually around $100 for the module.
The placement of the unit is a real unknown. I've placed power inverters for neon lighting right next to an amplifier with no noise whatsoever. With a device that delivers 10,000 volts at low current, I would've bet large on noise being present. In other cases, I've had to deal with noise caused by daytime headlight modules running on automotive voltage levels. I can only recommend that you try a few locations before you commit to mounting the inverter.
However, keep this in mind: You should buy a "pure sine" inverter rather than the cheaper "modified sine" style, or you run the risk of all kinds of noise infecting your system regardless of where you locate the unit. See if you can try before you buy.
Good luck with the project and sendme photos!