Jimny boot filled with water after driving through torrential rain (pics inc!)

Thanks for responding! I think he's right. Condensation leads to a baseline of water just "being there" and then dodgy seam sealing / only being spot welded means that if it doesn't outright leak into the cabin (like it seems mine is) then moisture still sits there and rots it out.

The solution would therefore be to properly seal the seams and apply insulating material or similar to reduce condensation forming. Then apply some sort of rust preventative stuff on the underside of the vehicle, maybe lanoguard.

It's telling that, aside from the jack area and rear corners, the next most common place for boot panel rot is around the seatbelt anchor points. Likely because its got spot welds but nothing to keep the water & salt from just sitting in the gaps between them.

At the weekend I removed the rear plastic panels and took a proper look. Went under the car too. My conclusion is basically unchanged from above.

Improper seam sealing can allow water into the car that then flows backwards as the vehicle is in motion, collecting in the rear corners and also in the rear jack tray. Depending on how well your vehicle was sealed this may be more or less of a problem, on mine it appears to have been an issue but I've now wire brushed the rust and sealed over the factor seams with Puraflex 40.

Then there's the seatbelt anchor points in the back which rust out along where the boot crossmember is welded. The water likely sits in the spot welds. This could be prevented by using a good creeping underseal like Lanoguard or Dinitrol if the vehicle is new / not already rusted.

Further, the idea that the jack tray gets heated up by the exhaust back box seems very plausible to me. I drove 40 minutes on a cold day and you can feel just how warm the panel gets. Not burny hot but enough to attract condensation I am sure.

The only thing left to do is drive through a bunch of puddles and see if waters gets in again... Even if it does at this point I will just accept it and move on. The amount of rain I went through to cause the aquarium in the back was crazy and we only drove in that weather to get home because we needed to be back for work!