I had the same problem the fist week i owned my spec v i sent it to Nissan
and they told me it was because the trunk was out of line
go and measure along the gaps along the trunk and you will see if its evenly spaced or if it is off. if not then your rear window might be missing some of the silicone sealer around the glass.
then bring it in and let them fix it up
and for now to ge the wate out you should pull a drain plug (dont lose it) and make shure you dry it up or eles it will rot out like if it had cancer.
i would even wax the inside just as a precausion
good luck:toothless