Instructions
Download and install the SparkIV modification software available at GTA Gaming. SparkIV allows you to make changes to your installation of "Grand Theft Auto IV." It also allows you to reverse any changes you have made at any time.
Download the Ford Crown Victoria mod for "Grand Theft Auto IV" from Softpedia.
Hit "Open" from SparkIV's main menu and select the "vehicles.img" file. Opening vehicles.img shows you all the cars currently available in your installation of "Grand Theft Auto IV."
Click "Export All" to send the list of game's existing vehicles to a folder of your choice. Drag the Crown Victoria mod into the folder where you exported vehicles.img.
Select "Import" from SparkIV's main menu to import the contents of the folder back into the vehicles.img file. The Crown Victoria will now replace regular cop cars in the game.