Mediafire usually presents you with an ad-screen, but you need to watch the upper right corner for the 5-second countdown, then press the "Skip Ad" button. When you find a link on a download site, it will either download directly, or you have to download via Mediafire. Often, just adding ORIGINAL to the end of the file will prevent it from being overwritten, and GTR2 shouldn't load it, but rather the new files you're copying into GTR2's directories. Also, keep the original versions of the files that you are replacing. To be safe, look up information on creating a "restore point" in Windows before doing anything that you think may cause problems. This is an "at your own risk" sort of thing, but if you follow the instructions, you shouldn't have any problems.
GTR2 MOD MANAGER MOD
You will need to search for a mod to "unlock" certain types of cars in GTR2. They usually identify problems, and whether the author has corrected them in an update. Ideally, you should go to sites where other users have posted comments and rated the mod. Also, some may be "questionable" considering their origin, or if they were "copied" from somewhere else. You can find some good mods there, but there's also some incomplete or poorly-developed mods as well. DrivingItalia is probably the least-managed of the download sites. There's a couple of good German sites with a few hundred mods. Do a Google search for GTR2 Mods, and you should find at least three or four sites. I'm not going to list any sites here, or promote them.