Photo of where this soft spot is?
The previous owner did a quick patch on it, as you can see. The damage was covered, but all I did was poke at the area in front of the patch with my finger and it pushed right through. That's the only real rust issue I can find on the entire car other than brake and fuel lines, which seems pretty good for a car with this many years and miles on it.
The more I look at it, the more I think the rust issue is from water getting in through the wheel well at one of the seams. Then the water just runs down to the floor right under the parking brake pedal, and sits there until it finally dries out on its own.