Something about this discussion rang a bell, a vague memory, so when I was out in the garage this weekend I had a look at my trunk, and somewhat to my surprise, found that I had these clips on both sides!
When I first bought my car (2000), I replaced the cardboard side pieces, and I feel fairly certain that these clips weren't in place then. My (now retired) mechanic Steve (Silver Star Motors in San Jose), who loved this car, was always doing little things to fix it up when I brought it in for service, so I suspect that he had these laying around and put them in for me.
Without this thread, I would never have thought twice about them.
-David
P.S. Sorry for the poor photos...