I just had the whole panel off, but I didn't think to look there... The annoying part to remove the whole panel is the safety belt bolt near the rear seat (not enough space there, you need a open wrench. Plus pulling the luggage area flat panels. See in the attached pdf the steps.
I would try to stick a light and a camera trough the side panel, maybe you see the connection type.
ID checked and its surely removable but I Think there is some clips or screws inside so it cant remove without remove the the side panel . If someone has pics from inside the panel, its should be great .
Best bet is to stick your cell phone camera in there with the flash on and take a pic. Toyota frequently uses plastic weld studs to hold minor plastic pieces onto larger panels, making them one assembly. This is hugely annoying but means it’s usually best to take the whole panel off, otherwise you might not get the small panel back on.
Finally I removed the whole panel. The storage bin is fixed with 4 screws and 4 plastic clips (See pics). Once the panel is removed, the bin is very easy to remove. So thanks to all for these info about how to remove the panel.