Well, knowing that there are loads and loads, and loads of cars lying around D.C., plus knowing that they blow up pretty well, which should mean that they still have power or gas in them, I`d just construct a buggy or rather an armored vehicle. Something that can go over large rocks, climb up hills, like the vehicle in Mass Effect, that can squish any living being in the way. Is ofcourse covered in a 10cm thick armor, so that nothing could hurt me while i`m inside it, with an anti-rad rubber layer from the inside so that radiaton wouldn`t affect me while i`m inside it. Equipped with 2 Mini nuke launchers and 2 Vengeance copy`s, with solar powered self replenishing batteries, for maximum damage. For additional coolness, i`d paint it black.
So I`d call it, Black Sheep.
Parts that i`d need are:
20 car bodies
8 car engines
2 Mini nuke launchers
2 Vengeance`s
6 Big wheels
+ all kinds of nonsense that i`m too lazy to write down.
There, did i just create the most badass weapon/vehicle in game history, or what?