Is it alright if I add you on skype? If you have one? And yeah they must be, because ive checked update.rpf , common.rpf , and i found niko inside of the part4.rpf. So yeah lol.
The default vehicle textures are in one of the "part" files from the disc. I cannot remember which one. But yeah, the DLC textures use a different format so they won't open in the texture viewer I don't think (unless he updated it). You could probably get it online by swapping the hash of the default character to Niko's or something.