I believe a common rust spot in the s13 is the hatch area and under the hatch lining where the spare tire would be. hatch leaks and water accumulates under there and rusts. I had (2) s14 5psds but my buddy has had about 10 240s combined. like 7 s13s and 3 s14s

 

I had to cut the trunk lining out and the piece of wood above the spare, which was molded to hell, and left the drain plug out......i also saw where the puddle is