Yup, fishing those bolts was definitely the hardest part for me. Don't remember how I finally got them in, but the design of that rail sure made things difficult.
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For posterity, and the benefit of anyone else doing this install, I'll describe how I fished the bolts closest to the rear of the vehicle, as those were the hardest and the instructions didn't help much:
I inserted the bolts and rectangular "washer" from the end closest to the rear of the vehicle. There is a hole at the top of the frame tubing. I fished the line through the exit hole, then brought the rectangular washer and bolt in.
At this point, the first thing I tried was to insert the bolt into the washer and just tug it toward the exit hole with the line, but that ends up binding the rectangular washer against the slanted piece of metal in there (see the diagram in the instructions).
Instead, I turned the rectangular washer on it's side and inserted the bolt. Again, this is all with both the bolt and rectangular washer inside the frame tubing already. I have very small hands. I can't imagine someone with large fingers doing this.
It's counter intuitive, and I can't quite visualize how it helped, but for some reason having the rectangular washer on it's side with the bolt inserted allowed me to tug the line and get the bolt to come through the exit hole without the rectangular washer binding on anything.
Hope that saves someone an hour. It was very frustrating.
There may be a better way to do it. This is just how I ended up getting it done. Good luck!