I'm no expert but....
1) Use Sharkport and "back up" your garage file from slot #1 mem card.
2) Close Sharkport program.
3) Open Garage Editing program
4) Click "read garage", select the garage you just saved using Sharkport
5) Now you can select any of the existing cars in your mem card save and manipulate parts, even select the "hidden" cars available. Try experimenting by Right clicking on the engine or suspension and create a hybrid.
6) When finished, click "write garage" and overwrite your last save. close Garage Editor.
7) Now open Sharkport and upload the recently edited save onto your mem card slot #1.
DONE. [just practice a few times, this process becomes very easy, now you just have to keep all the new garage files organized!]