Hey Allen,
For some reason, I didn't catch this post till a little while ago. Then, I went to the garage to take pictures of how it works. Once I got down on the garage floor, I couldn't stand back up.

Hence, the delay. O.K. here's how it works:
This is what you're looking at now, right?
The '03/'04 vent has a little curved tab on the lower front corner. It holds the vent to the front and bottom. It looks like this...
When you put the new vent in, put the rear in first. Once it's in place, slide it Back as far as it will go. This will give you enough room to drop that tab behind the sheetmetal.
Once it is flush with the body, move it slightly forward and down so the tab goes behind the quarter panel at the front and bottom of the opening. Put the screw in the top and you're done.
But... while I had mine off, I put some mesh inside of the vents. I've heard horror stories of desnorkled Boxsters sucking up schmegma, particularly lit cigarettes, so I did this...
It's held on with tiny wire ties at the corners (man, that's a bad picture, sorry).
Don't worry if you don't like the mesh look. Once the vent is on the car, the mesh is nearly invisible.
Hope I answered your questions, pal.