I thought I'd follow up in case others with the large rear window are thinking about doing the same thing. As near as I can tell, it's not at all practical to place the mounting plate where I drilled the hole above. There are too many layers of steel behind it to get the plate into place. Instead you have to go around the corner, even though this doesn't seem like the straightest route for the seatbelt to follow. We did both belts mounted in this position and they work well (see photo below). Now I just have to patch the exploratory hole on the other side and figure out where to store the jack, which was displaced by the seatbelt retractor. I hope this helps!
