[71Corolla]

Looks to me like the seal around the vent is failing. If it was me I would hit the wreckers to see if you can find a better one. Failing that, get creative and use something from the weatherproofing section at the hardware store. They sell flexible putty like strips sometimes called "crack seal" which will work pretty well. You could also use a polyurethane sealant, but not the best solution because removing the vent for any reason would be difficult or impossible.

edit - that's a nasty environment for rust to breed, I would clean that up with soap and water, then something like CLR to further clean it. Then spray it down with a rust preventive oil and leave it like for a while, it will drive the moisture out. I use this product it works really well.

 

[Jigg007]

Could use a gasket maker or RTV sealent. If you would have to remove it its easily cut with a knife. Clean down the area and all that good stuff before you apply it, of course.

If you cant solve that, you could paint the inside of your trunk, and put a drain hole that the lowest point Perhaps lube it up with a thick oil or wax to really run that water. But sealing the leak is the preferable alternative.

 

[Chunkywolf]

I'm considering buying a new vent since that corner of the car was hit in a crash my sister had about 2 years ago, and the vent may be cracked itself. It seems that a new vent would include a gasket or seal with it since there's no separate part number for a gasket.