Holy thread resurrection (Well... sort of...).
I've just stumbled across this section today, so bear with me.
The only JDM80's that had the 3rd-row seats were the wagon/gasoline models. So unless the PO ordered from Toyota the 3rd-row seats put in when ordering new they didn't come with them. I have only seen maybe two HDJ's with them in. It's VERY rare indeed.
It's not exactly just get the seats, seat belts, and the brckets and slap them in/on there to install them.
You would have to cut four holes in the floor, put/weld the hook thingys to hook the seats onto the floor with, weld the bottom of the said holes, buy the inner tire fenderwells (both sides) and cut the brackets that the seats attach to (where it folds) and weld them to the existing fenderwells, attach the seatbelt pieces (holes and stuff are pretty much there like for the 2nd row 3 pt seatbelts), and you're done!
Before I looked into it seriously I thought I would have my son's soccer buddy's father who owns a body shop put them in, but as you can see in the pics it's a little more involving than just shoving them in there.
Anyway, I'm sure as with just about everything, you can fab something up w/o using OEM parts, but I'd like to stay as close to OEM as possible when it comes to this kind of thing (Don't ask me what exactly "this" kind of thing is

)
I'm hoping that I can con the shop owner guy to install the seats sometime over the winter holidays (I have asked him to go ahead and order the fenderwells and the hooks, etc...) Don't hold your breath though.
Pic 1: My rig with the matching 3rd-row seats that I picked up on auction.
Pic 2: Right side
Pic 3: Left side
I probably should wait until posting these pics, but ...