Sorry, I've been out of town on business. I will take a couple shots when I get home this weekend, and of where the leak is coming from. The good news is I found the part number, so now it's just ordering it and finding time to install it.

 

I had a trunk leak in my 2007 Saturn Ion, and it seems similar to the one described above because the only part of the trunk that showed clear rust was below the vent (see pic 1). Here's what I did:
1) I went to LKQ and found an Ion with no visible rust below its vent, and removed the vent (~$2 + $3 entrance fee).
a. It isn't necessary to remove the entire quarter panel of the car to get the vent on/off, you just need to remove the plastic shroud (see pic 2) in the rear passenger-side wheel well.
b. The shroud comes off easily once you remove the 9/32" bolt and 7 plastic fasteners in the wheel well (see pic 3).
c. I wouldn't advise anyone to try to pull the vent through the trunk. It looks like it might work, but you could also damage the seal and it is just too easy to get it out through the wheel well with no chance of damage to anything.
2) I ran water from a garden hose onto the rear passenger quarter of the car for about 1 min, and observed that the leak was only around the seal and not via the flappers or anything else (see pic 1). This is not completely authoritative, but it was a good indicator that there was something wrong with the seal in the vent or the hole for the vent, rather than anything else.
3) I then removed the old vent and compared the two vents. it is clear that my vent seal was much more worn and pinched than the seal on the LKQ part (pics 4, 5, and 6).
a. The LKQ part's seal was not pinched and it was well formed all around the vent.
b. The LKQ part's seal was not disintegrating anywhere.
4) I inspected the hole where the vent goes, the metal was smooth with no signs of deep rust or damage, just some surface rust (pics 7 and 8).
5) I dried the area, inside and out, waiting an hour for it to be bone dry before replacing the LKQ vent.
6) I ran water on the rear quarter of the car for 2-3 minutes and observed no leak (pics 9 and 10).
7) I didn't put RTV/silicone on the opening because of the differences I observed in the two seals and the fact that the opening for the vent was the correct size, undamaged and had only surface rust. The seal disintegration could be a result of a more rainy climate than the LKQ car, but not the pinching/twisting of the seal (pic 11). I believe that happened at the factory.
a. Putting RTV on the opening would have required me to remove the bumper, or rear quarter at a minimum, but I will do so if I observe leaking this Winter or after a wash.


Original vent leaks after ~ 1min with garden hose on shower mode. Leak is from the vent seal.

Rear passenger side wheel well shroud fastened with 1 9/32" bolt and 7 plastic fasteners.

Rear passenger side wheel well shroud.

Note deterioration of original vent seal (top). LKQ vent is below and shows no deterioration in its seal.

Note deterioration of original vent seal (top). LKQ vent is below and shows no deterioration in its seal.

Original vent on left, LKQ vent on right.

Vent hole edges are smooth with only minimal surface rust.

Vent hole is undamaged.

New vent did not leak.

Rear trunk vent does not leak after 2-3 min with garden hose in shower mode.


Original vent on right, LKQ vent on left.

Note pinching of seal in original vent

New vent did not leak.

 

Congrats,

I believe you just set the record one our homely little forum for a forum member with the fewest total posts that includes a sticky.
Nice writeup - and 1 pic = 1000 words