I'm following up on my own post in case anyone is searching for this information in future.
The lamp holder is in two pieces. The piece with the electrical connectors comes out the top when the tiny tabs holding it in are spread enough to withdraw it. The attached photo titled "Boot lamp top.jpg" shows the ridge of the tab toward the top left of the fitting. The plastic is very rigid, so it takes a bit of effort to spread the tabs enough.
Once that is done the two large tabs at the bottom of the holder need to be pushed in to drop the remaining piece through the hole into the boot itself. Again, it's very rigid plastic, so care is needed not to snap the tabs. I got one side started in the hole (visible on the right hand side of the photo), then pushed the other tab in while pushing on the whole piece from the top.
Installation is the reverse.