Right, Open TDU Modding Tools, Click "Patches":
Then click "File" > "Open Patch".
A new window appear, select the file you've download who's inside the .rar
Then click Generate and Select your main Test Drive Unlimited folder in /Programes Files/Atari/Test Drive Unlimited.
After that, Test > Run Patch
A new window appear, Click on the blue arrow. Just like a normal car installer.
Then you are done ! :) Patch is installed. But it's not completely finish yet.
Remember, all the cars set to the dealers will disappear. So what you have to do is reset them. Open Vehicules Manager > Select Any car by double clicking > select dealer > Scroll down in "1", select any dealer > select any free slot, that's 2 > click on set, that's 3, and click SAVE !!! That's 4. Done :) Easy !
And the great thing is you can add so many more cars in the different dealers. Like Dodge, in the past you could use only 2 slots, now you can use 7 ! Isn't it that beautiful ;) Enjoy !