Areas that rust - Snorkel box
This is behind the tail-lights... (Syncro version shown)
Notice the water that drips in through the air-intake vent grill above, and can end up damaging electrical units (on injected petrols), rusting the seam above the chassis, dripping down into the rear box-chassis and rusting its bottom capping plate away. (It gets in there mainly through the hole (pictured) at the left edge of the snorkel box's floor)
The front face of the box where the filler tube goes, also seems to corrode badly, but is non structural and can be plated with thin sheet/pop-rivets/RTV silicone - ditto the flitch plate that runs forward along the inisde edge of rear chassis (n/s and o/s)
One solution is to at least drill a hole in the centre of the box's floor where the pressed fingers cross... so water can drain down and out. Deburr, swage outwards with a suitable tool (helps prevent splash-water ingress), anti-rust-prime and paint.