First you must either have a DDS plugin for your painting app (available from the Nvidia website) or a DDS converter to convert the files to a format you can use and back again.
The files are all located in your LFS/data/dds folder. Anything with "interior" in the name is generally an interior file, although there are some more obscure names. There are also the universal textures, like the shifter knob and the shift gate cover. Have a look through your dds folder and you should find what you're looking for.