From the Options menua, highlight Game Status and press L + R to access the Password screen. Now enter the following passwords:
To unlock bonus characters, you have to complete all the objectives in Quest mode with one or more other characters, as per the below chart:
Change Vehicle View
Hold B and press Y.
Ghosts in Louisiana level
Destroy the graves in the cemetery. Ghosts will appear and begin attacking.
Flood Louisiana Level
Approach the bridge (or dam) and shoot at the nearby wheel. It will start to spin and the bridge will begin to raise. Keep shooting until the bridge is completely raised. Repeat this at the other bridge. In a few seconds, it will start to flood.
Triple Salvage Points
Destroy your opponents with a special weapon to get triple salvage points.