My gasket was in pieces when my trunk was leaking, but I got it sealed up without going to the outside.
The vent itself is able to be removed from the inside. I think there is a springy plastic clip (it was a year ago, I don't remember too well) or something of the like. The I put on A LOT of silicone and put it back in place. Then more silicone around the edge. It didn't look perfect, but it's hidden so it doesn't have to.
