Instructions
Download and Install "San Andreas Mod Installer" (SAMI). The program is available for free from FileFront and can be installed by agreeing to the program's "Terms of Use" and then choosing a directory to install the program. Any directory will work fine.
Download a vehicle mod for "Grand Theft Auto: San Andreas" that has a converted car file from "Grand Theft Auto III" in it. These files are abundant and are found on a variety of file-hosting sites, including FileFront, FilePlanet and other fan sites dedicated to the game. Once downloaded, the mod will be in a compressed format.
Start "SAMI" from the start menu or by double-clicking its desktop icon. The program will automatically open to its "Mod Loader" screen.
Click the "Browse" button in the "San Andreas Directory:" field to search for your installed "Grand Theft Auto: San Andreas" directory. This will vary depending on where you instructed the installer in the installation process, but the default file path is "Program Files\Rockstar Games\GTA San Andreas." Once you click on the directory, click "OK" to close the window.
Click the "Browse" button in the "Mod Location:" field, and then navigate to your downloaded mod file. Click on the downloaded mod file in its compressed format, and then click "OK."
Click the "Continue" button twice. This will take you to the vehicle replacement screen.
Click on the vehicle you want to replace with the converted car from "Grand Theft Auto III." Click "Install." The next time you start "Grand Theft Auto: San Andreas," the converted car will be shown in place of the car you replaced it with.