Son had similar in his magane look for drain holes along boot seals gutter if there are any they may be partially blocked. just a ' maybe'
xxx is off the mark as you clearly stated BOOT now another 'maybe' is the rear window seal wipe vaselene(petroleum jelly) around the seal all-round both where it's fitted to frame and glazing. It's a clean grease easy to clean off.
bob
February 2016