I finally fixed the rattle that's been aggravating me for the past several weeks. User SaveChief found the source of the rattle
in a previous post, but didn't update his thread with a how-to or any postmortem. I'll do that here.
The fix is really easy (requires no tools at all), and I wasted a lot of time messing around with the passenger-side vent and the cupholder assembly trying to find the source. The rattle is indeed where SaveChief thought it was, where the plastic vent tube leading from the passenger-side vent connects to the ventilation system. The tubes become disconnected, and the vent-side tube rattles against fixtures behind the glovebox.
To get to the rattle, open the passenger door and pull on the bottom of the trim piece to the right of the glovebox. The piece will pull out fairly easily - it's only held in by some tension clips.
Once you've pulled that piece out, pull out the the plastic trim piece to the right of the piece you just removed. It attaches by two tabs that slot in behind the rubber door seal, and a tension clip at the front of the item. Pull it out of the tension clip, and pull toward the back of the car. Again, this item pulls out fairly easily.
Now you'll be able to see the source of the problem. The black plastic vent tubes that connect to each other right behind the aluminum support brace will probably have come loose from each other. Simply push the vent tube on the right into the one on the left securely. Then use some duct tape or other solution to seal them up so they won't come loose again.
At this point, I also wrapped some felt tape around the wire that hangs in front of the vent tubes, as I felt (ha!) that it would also help prevent any potential rattles. You can just pull the wire out a bit in order to get the tape on it.
So that's it! Just snap the two trim pieces back into place in the reverse order you removed them, and you're done. So far, my rattles in this area are gone and I'm much relieved. Also, I know that if I hear the rattle again I can get to it on the spot with no tools needed.