Do You Know This Book?

People, I need your help! There is a book that was about when I was at school that I would like to read, but I can't remember the title or the author. A friend of mine at school read it, but I wasn't really interested... until now.

So what I know about the book. It's a young adult novel. I believe it's a mild horror, possibly a mystery. A girl is going off on a road trip after splitting up with her boyfriend. On the drive, she sees two hitch hikers, a guy and a girl, about her age that she picks up. They're either a couple or siblings, I can't quite remember which. As time goes by, I believe they run out of money, and there's still a long way to go, so the guy persuades her that they should rob a shop - I think this happens a few times. In the end, you find out that these two hitch hikers are actually ghosts. I don't know the main plot of the book, as I never read it, I was just told about it, and read excerpts. This was about ten years ago. I could be wrong, but it may have been on an award's shortlist, because I think that's why my friend was reading it, she was reading a number of books for the school library for an award and this book may have been one of them. I believe it was the Carnegie award, but again, I could be wrong.

Does this ring any bells with anyone? If so, please let me know! I want to read it! Thank you! :)