You may want to check and make sure that your back window is ALL the way up.
Also if you disconnected your battery at all it will do that. Roll the back window all the way down and hold the button for a second past then all the way up and hold the button down for a second past when its done. This will reset it.
There really needs to be a sticky about how to make the truck re-learn the "fully closed" position for the rear window to enable all its features, as well as the auto-up driver's window and moonroof features. This is probably the most common question on here and it's such a simple fix.