In your lfs folder there is one called DDS which contain the textures for the interior of the cars. If your going to modify them then you will need to get a dds converter (google for it) which converts to jpeg usually so you can edit them in whatever program you like. Then you can convert back to dds files put them into your dds folder with the same name as the original (alway good idea to back up the original file though just incase) and see them in game.
Also if you change the dds files you will change them for all cars that use those textures (dont think this applies as your demo racer though)