Darren, as can be seen in the pictures on that link, the wagon has a hole at the top of the outer cowl once the fender has been removed to do a visual inspection. (one retaining screw in the cover) Unsure if there's a similar one on the truck, perhaps someone else can chime in.. This access permits inspection to identify any issues inside, and to clean out any debris that exists (before it would have become an issue) Yours likely was caused from debris collecting inside.
