I saw it so long ago it's difficult to remember. It's a French film with a female lead role and it involves a serial killer who we're first introduced to driving a truck of sorts after he's finished skull fucking a head. If you've seen the movie, I'm certain you can't forget it. Plot ends up being the girl as the serial killer but being unable to remember anything else about the film makes me want to understand the memory I have of this movie. It has to be within 14 years recent, psychological thriller/horror/slasher?
Is it High Tension by any chance? Sounds similar to what you described. http://www.imdb.com/title/tt0338095/