I figured out what is going on. I'm becoming convinced that Magilla Gorilla has done work on my car. The Rivnuts take a 5/16x18 (course thread) bolt, but Magilla decides to use a 5/16x24 (fine thread) bolt like the ones used to bolt down the seats. So of course the bolt cross-threads and jams. Magilla twists harder and strips the Rivnut in the body - not once, but both of them. One fits snug, but spins. The other has quite a bit of play in the hole.
I tried using the homemade Rivnut tool, but the nuts are already compressed and won't tighten up any more. I think the spinning Rivnuts have reamed the holes out too much for new ones to work. I have a welder and the console and carpet are out, so I can tack them in place. Thanks for all the advice.
