Virginia,
I spent considerable time re-caulking the outside areas of our unit last summer. The center, outside strip covering the laminated seam on both shells was in good condition - except for areas of loose caulk. I would suggest making sure the area is free of old caulk and clean. Adding new caulk should be sufficient to seal the strip. There are several other possible sources of water infiltration. In my case, both roof vents had slow leaks. However, re-doing the caulk around both vents did not stop a persistent leak in the upper shell, by the ceiling cabinets. I finally corrected that leak by caulking (or replacing the caulk) all along the corner molding covering the area where the roof meets the side walls.
https://photos.google.com/album/AF1Q...vH78GkGZIXd7L3
Dick