[psomers]

Had the same problem with my 2013 Mountaineer. Remove the vertical corner trim and you will see the problem. The side wall comes up short about 3/8” of being flush with rear wall. The angle trim only covers about 7/16” and there is no butyl tape on side.
There is a short piece of 2x4 in bottom corner that I had to replace. Fix for leak, at least I hope, was to use Dicor corner seal. Cost for seal kit about $100. Do search on Dicor corner seal. Also on you tube.