Please revisit your manual. The top strap should be under the head rest not over it. The top strap should also wrap around the top part of the car seat not be coming directly from the middle over the top of where bub would be. Also make sure the passenger seat is far enough forward that it not making contact with the car seat (bit hard to see in the video but it looks very close).
@@lundyale -.- How did you know! haha. It took me a little to long to realise I could use the support of gravity to get it through. But now I could do it in a minute.