I took my 03c into the dealership to have them fix a rattle comming from the rear parcel shelf (passenger side). This rattle was especially annoying, because it would vibrate on certain bass tones, as well as over medium to large bumps, and it was pretty loud.

Got it back from the dealership, all good. Until two months later, when it came back as bad as before, and of course the rattles were no longer fixed under warranty. After a little fighting with the dealership (who imo never fixed the problem so much as they temporarily alleviated it), and since my rattle warranty is up, I decided to tackle it myself.

Here's what I did (note all the pics are thumbnails that link to larger pics if you need more detail).

Drop the back seats down, and remove the finishing piece around the latch (just pull it straight off).

Remove the rubber trim lining the opening from the trunk to the cabin.

Underneath the trim, you'll see three metal clips holding the trunk trim flush to the seat backs. Remove the clips by squeezing and pulling.

Remove the plastic clips holding up the trim inside the trunk (these are the clips you remove by popping out the inner piece, then just removing with your hand)

Now drop the trunk liner down (mine was taped up around the trunk lid, so I just let it hang down).

If you lie down over the folder seat backs with your head into the trunk and look up, you should be able to see three separate things that I found rattled when I tapped them.

First, the trunk light. This guy fits loosely into the metal frame, and has a dome that also is loose... So I wrapped tape around it, under it, and a little extra to make the dome fit tight.



Keep looking up - and you should see this



The plastic clip there rattles when I tap it. So I put some weatherstripping tape under it to fix it.

The circular hole in the metal also seemed to contribute to the rattle - seems like the shelf is vibrating against it - because whenever I stuck my finger in there and tapped from the top, no rattle. So I cut a little foam circle out and shoved it around.



Giving me this



After all this, I think that another possible thing that may be rattling is the wire running to the little trunk light I taped up earlier. To fix that, I removed the two plastic pins holding down the rear parcel shelf, and then lifted it up.

This is what it looks like underneath



To understand the pic, you're looking into it from the middle, just above the seat latch. You can see foam to the left and right, and straight in front is the wire for the trunk light, and the newly taped up trunk light itself. You could probably tape down the wire, but what I did was just cut some more foam to fit, and slide it all the way down so that it was between the wire/light and the parcel shelf.

Then just close everything up, and enjoy one less rattle.

Now if I can just figure out how to make the seatbelt not rub against the seats...