Patina???
patina is what you get when something gets old. look at antique furniture. some is restored to perfection. some is showing its age and use. rat look (to me) is about patina, rather than forcing your car to look like a pile of junk.
pull a golf out of a barn, clean the lights and windows and stick an mot on it = rat look.
getting a decent car and stripping the paint off the bonnet < rat look