any file that ends in .car weather it is .cup.car of .gns.car is an in-game car file. You will to open the texture file within the car you need to first extract it either in the game which will save it to you imports_exports folder or by using a win mip 2. both meathods save the texture file as a .tga.

By the way, if you modify a car someone has already done, don't post it as your own work. Contact the painter, ask them if it is okay to post it and then give them credit for the work they did. Nothing will make people madder faster than someone ripping their work.