rear hatch weatherstrip

first happy thanksgiving to everybody, I do have one question maybe someone can answer. I have been chasing exhaust fumes which really are not that bad, but the wife still smells it. on my rear deck I have the large weatherstrip that's been installed years ago but I was missing the small one that I call the part where it goes the rain gutter. Nissan part is a 3-piece weather strip. the one msa sells made by precision that is a 1-piece weather strip that glues to the top channel where the hinges are and down the 2 sides. I really do not know what is does, is it meant to keep water out of the channel or is for exhaust fumes? thanks