You know, this is a REALLY good question! Even though it's one of those that is directly stated in the book, it is glossed over so very much that one doesn't notice the answer. It's just the kind of detail that can help to solve a crime investigation, . . . but that no witness can seem to remember.
That being said, the answer to your question is that Mr. Harvey (Susie's killer) drives an early 1970s white Jeep Wagoneer. (The only thing that needs to be estimated is the exact year because it is not stated in the text. Seeing that Susie was killed in 1973, an early 1970s model is a good estimation, as would a late 1960s model.) Here are a few places where the model is discussed (the make just took some simple research on my part):
[Mr. Harvey] whistled inside his huge station wagon and congratulated himself, he felt full-up. Apple pie, cheeseburger, ice cream, coffee. Full. . . . It was late when he got there, and he left the safe in his Wagoneer while he approached the house of the Flanagans, who lived on the property where the sinkhole was. (51-52)
The irony is, I find this kind of information to be incredibly important, . . . not necessarily to The Lovely Bones, of course, but in real life. How many times are witnesses unable to remember the simplest details of a crime scene? I distinctly remember an episode of Punky Brewster (a kids TV show in the 80's) where the teacher staged a fake robbery. Even though the perpetrator ran into the classroom and grabbed a purse in broad daylight, NONE of the witnesses could remember anything about what he looked like. That always struck me.
Back to The Lovely Bones, if just ONE person had noticed this car, this 1970s Jeep Wagoneer, in the strangest of places (at sink holes, at construction sites, etc.), . . . perhaps Susie would have gotten some extra sweet justice.