Its easy do this.

1. Download the files found below.
2. Have obviously GRID 2 installed
3. Open the downloaded file
4. Open the GRID 2 folder where you can find the Grid2.exe or what ever the executable is called
5. copy & paste all the files located inside the downloaded file
6. Let it overwrite everything it asks to.
7. start game
8. voila all DLC cars should be unlocked.

Just open the .rar folder
Copy all the content inside the .rar folder
paste it into your directory where the game is installed (where the .exe) is located
and let it overwrite anything it asks to overwrite.

(Note that the gdrive file is not mine, Something i found on YT & from another now removed guide. Malewarebytes throws an warning for it so do it at your own caution(?) Again NOT my file so i'm not responsible for it.)

Seriously. Stop giving key seller websites money. Nothing goes towards Codemaster either way & the game & DLC's are no longer on sale so its counted as abandonware.