2013 4runner 3rd row
I settled on a trail with the idea of swapping out the rear cargo tray for a 3rd row. I wanted a clean factory look and not the aftermarket look I had used in my 4th gen. I was happy to find out the swap is fairly easy if and this is a big "IF" you have a donor 4runner with the 3rd row.
I'll apologize ahead of time that these pictures aren't as detailed as I'd like if anyone is thinking about doing this swap feel free to contact me for more info. A couple notes, the best way cost wise is to find someone local to you and offer them 500-1000 to swap out the rear cargo for their un-used 3rd row. I did it around Christmas too so that may have helped

You also have to let the person know that their rear end won't be completely stock looking but for the donor I used he was happy with the results.
I will get some better pics of mine soon but unfortunately I believe I lost the pictures of the donors rear end after the swap.
Here are both stock rears.
2015 4runner with tray
You pretty much start by removing all the plastic (refer to manual because I don't remember it all
3rd row 4runner
Toyota did something weird on these, the rear seats literally bolt in, the holes and threads are all there. Well except for these which you lay the seats in, mark them then remove the seats and drill and boom you have threads. Thanks toyota?
The seat belt and the rest of the holes are there except the seat belt anchors. For some reason they put the holes in and the threads for the pretensioner but not the anchor? and it couldn't be in a more hard to get to place. We made a tab with a nut on the end of it that we had to fish into the piller and once the bolt was started it locked so we could tighten it. Kinda a PITA and the hardest part of the swap.
We also had some no so pretty cuts to make in the side panels, which you need to keep yours unless you plan on swapp out the rear seats and everything else. We didn't opt for swapping the seats, they are completely different and we weren't sure if there were even holes for them. Plus our doner 4runner had cloth and we have pleather so we opted to not swap them and cut plastic.
All the cut plastic is barely noticeable and can't really be seen once everything is installed.
I will come back to this later and add more pics of the finished product. I also need to get the seats recovered to match the front seats but they are black cloth for now.