Instructions
Download the "Need for Speed: Carbon" Trainer (see Resources), and unzip it to the installation directory of your "Need for Speed: Carbon" game.
Start "Need for Speed: Carbon," and then press "Alt+Tab" to switch back to the desktop. Open your "Carbon" installation folder, and then double-click on "chnfstrn.exe" to open the Trainer. Press "F3" to unlock all the cars and parts, and then "Alt+Tab" back into the game.
Go to the menu of "Need for Speed: Carbon" and select "Custom" or "Bonus." Every bonus car in the game should be unlocked here, including the Pagani Zonda.