They were poorly welded there at the factory and I've seen gaps packed with seam sealer. The sealer may have been removed (or never installed). Panel bond nicely fills it and makes it stronger. Dig out any sealer, clean any rust or pitting, and pump in panel bond then wipe with your finger into the groove. Be sure to wear a latex/nitrile/whatever glove or the panel bond will be a pain to get off. After panel bond is dry sand with 80 grit to rough it in, then 180 grit and then prep/prime for paint like any other area.
You can see the black panel bond on the sides (top pic before sanding). Very little panel bond is noticable after sanding (2nd pic) and once primed (3rd pic) is undetectable. The panel bond locks out moisture so if the area is properly cleaned it works great for this area.
Any possibility the car was poorly repaired there long ago and the rear panel was replaced?
