I agree with disliking the ending. I was the exact same way, went back through the movie and triend to make it make sense. There are some parts that it makes a lot more sense after seeing the ending, but also makes other parts seem pointless. (Like you said, the truck.)
That said, the scene with the killer in the truck at the beginning made me extremely uncomfortable. That set a tone for the rest of the movie, so I think it did it's job not as a plot device, but creating atmosphere.
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