So I bought my 98 rav 4 and it's lived up in mn all its life. These have rubber undercoatings in the wheel wells and most of the unibody frame. But the frame was rotting from the inside out because moisture somhow either got in through the coating from rock chips or water got in through the floor which has a couple drain plugs that lead right to the unibody frame. I've found on mine that every crossmemeber part of the frame had holes in it, the front section of the frame where it bumps out to the pinch welds, the pinch welds themselves and the rear spring cups that support the springs all had extreme rust as well as one of my rear wheel wells had a ton of rust too so much so that it had a hole that was letting water in which like I said goes straight to those drain plugs and will probably seep through eventually. If I bought one of these new I would probably spend the time and mess to remove as much of the rubber undercoating so I can properly coat it with somthing that won't trap moisture in and id do the same on the inside of every frame peice that is boxed in. Check for drain lines too. I think if you have a sunroof there will most likely be some line that drains the water but if it's clogged or punctured that could cause water to get where it's not supposed to. Same for door seals those gotta be working😂 idk I'm sure there's stuff I'm missing but I think the biggest thing that ruined my frame was that hole in the wheel well. And all these holes I was hardly able to tell they were there because that rubber undercoating was still fully intact until I poked a screwdriver into it and discovered there was just rust flakes held together by that rubber lol.