If you mean the main back winfow, keep in mind there are two panels of glass, so you have 4 sides to clean. The innermost side is pretty easy - just put the seats forward. Then lift up the tailgate to get the "middle" two. Wear soft clothes that won't scratch and pre-wet a paper towel with glass cleaner to reach in from each side. Long arms help! Then finally lower the tailgate to get the outermost.
If your arm alone isn't cutting it, I would recommend a microfiber wand, like this:
Stoner 95161 Invisible Glass Reach and Clean Tool:Amazon:Automotive
"Invisible Glass" is also the cleaner I would recommend.
If you mean the two small side windows, a long arm or the wand is also the way to go.
Hope this helps.