according to the my window price tag sticker 1989 was :
235/75r15 all season radial (x5)

since we are so far from original question,this wont matter... what's the deal on a third seat? there are hooks for it. Does that mean the back seat can be moved to the rear, or is there another seat?

I have my tools in the way right now, or I would go look for myself. maybe the owners manual will say

1989 5 door montero 3.0 auto
stock original Japanese
about 200k miles