How to rid of these stubborn spots, for good. The tinted windows in the back show worse than the front, but can still see them in the front as well. Any ideas?
If they're actually etched into the glass (unlikely) then a light polish with some 0000 steel wool or some cerium oxide will be your next best bet. Otherwise, the suggestions above me are pretty good.
I use Zaino Z-12 Clear-View Glass Polish on my windshields about once a year. Even when there aren't obvious spots, it is amazing how much clearer the glass looks afterwards.
If clay barring or using a white vinegar solution does not work, I'd use a glass polish (many out there) or a water spot remover (Car Pro and Mothers have them, to name a few).