Will the glass pop open using the lower button on the key fob? Assuming your wagon is programmed to do that; the button could also be programmed to pop the entire tailgate.
If the glass pops open using the key fob, then the switch/button on the glass likely has gone bad. They're not too expensive, maybe $40, and pretty easy to replace.
If it's not a bad button but the fob will still pop the glass, you could have a wire issue. The switch's wire could be severed where it passes through the glass (behind the wiper pivot), or it could be broken up at the tailgate hinge where the bundle is wrapped in rubber boot.
There's a remote chance the glass lock actuator under the windowsill of the tailgate could be bad, but most folks seem to have button or wire issues.
Last edited by Germane Riposte; 12-31-2015 at 08:22 AM.