As with all missing texture problems the part where you have to do some research can't be skipped. But making it easier for some people who dont want to do some research or dont want to download the svn here you have the addon that includes the missing textures for the interior.

http://steamproxy.net/sharedfiles/filedetails/?id=227607345

And if you wonder why the skin of the car or anything else on the car has no texture ... then there is no way to help you anymore