1st rust sighting inside and some distortion

[Ivan141]

If I we're to hazard a guess, I'd say this is the left rear mount (near transmission tunnel) of the passenger seat (or drivers seat for rhd cars).
It's the exact same thing that happened to my left rear drivers seat mount. The bit that got torn out should still be attached to the seat rail if you're lucky.. I would hammer the bits back up so they are a bit straighter again, then weld the torn out bit back in with butt-welds on every crack you notice.. then grind it flat and weld another reinforcement plate over it with a hole drilled out in the middle so the bolt can pass through. The nut is only stressed when it is being pulled upwards, since the rest of the mount takes the downward forces. Putting an extra plate over the mount is a good way of spreading the load on that tired old mount.

If you've lost the old nut, then you'll need to find yourself another steel nut with the correct thread, clean up that hole, and weld the nut underneath a reinforcement plate, then weld that over the broken mount. This will raise your seat by the thickness of the plate that you use, but it will not break again as easily (and I dont even notice the 1,5 mm on my car).