Rear Facing Baby seat in 3rd Gen 4runner
When securing my 2 rear facing car seats for my kids I used the posts/hinges that are under my rear seat. There is a plastic piece that is covers it but those are easy to remove.
When attached the belt from the very top of the rear facing car seat runs slightly diagonal toward the center of the vehicle where the closest post/hinge is located.
Hope this helps. My 4runner is a 2001 so I hope the post/hinge is the same on your truck.
I also agree with getting your local fire department to help you if you're worried about it. I am a firefighter for the city of Phoenix and our department offers that service by appointments. The firefighters that are certified to install the car seats take over 40 hours of training so that they can understand the different nuances to all the different cars and car seats. Also, take the time to read the car seat manual. Many of the seats are made in other countries and when they translate the instructions into English some do better than others

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When you're securing the car seat to the actual seat belt of truck, get a knee in the car seat and push it down while tightening the car seat so it is locked in tight, otherwise they move around too much.
Also, as a previous poster suggested, I suggest mounting the seat in the middle of your truck (if you only have 1 kid). This way in the chance of an accident any intrusions by another vehicle hitting you from the side is as far away from your child as possible. From what I've seen at the many car accidents I've responded to, any extra space, even a few inches... can change the outcome dramatically.
Good luck, congrats on the new addition to your family.