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You can get this from gta4-mods.com. There are several programs that allow you to install mods, but this program is the simplest way for cars. See this guide for details on extracting files. In most cases, you can simply double-click the downloaded ZIP file and copy the contents to your desktop. In the folder that you extracted, you will find a setup program. Run it to install the mod installer. Most users can leave all the installation settings at default. Your computer may warn you that the file is unknown, but it is safe to proceed. In the field at the top of the window, you should see the path to your GTA 4 game directory. If it was not detected, click Browse and set the path yourself.
Download the "GTA IV Vehicle Mod Installer". Extract the installer file. Run the installer. Open the program. Ensure that your GTA 4 directory was detected.