The Vigilante God has stashed every manner of weapon in each of the arenas. The trick is to find them and use them. You know what it is, and simply have to drive over it to attach it to your ride. You will not know what you get until you drive through the crate.
Green crates are the special weapons that are vehicle specific.
Always grab green crates-they hold your most powerful weapon. You'll also want to keep these out of the hands of your enemies. Your vehicle can hold up to three weapons at any given time. Depending where they mount on your vehicle, certain weapons will knock others off. Know which weapons fit where to maximize the impact you get from multiple weapons.