[Raises hand] I sealed it from sucessfully from inside[knocking on wood]. The gasket piece cannot be purchased separate, you have to buy the vent as well IIRC...wasn't it like $70? You
should remove the bumper if you want to do it right and avoid potential problems down the road, but it can be done from the inside. Good luck. I have fixed leaks in 3 different locations in the trunk, all on the passenger side.
