There are metal arms extending from the center handle mechanism to either side of the tailgate, which activate the spring-loaded latches. The arms were originally covered with cloth for rigidity, but the cloth frazzles and the metal arms get bent anyway. I solved this by removing them and having 1/4" rods spot-welded on them.
Anyway, to solve your instant problem, just open the window all the way and reach down with a long screw driver and trip the latch; that is, mimic the action that the metal arm would cause. Take a look and you'll see what I mean.
Once you've got it open, get some WD40 and spray it in there, something you should do periodically. But, in addition to strengthening the rods as I noted above, there is also a tendency for the inner tailgate panel itself to fatigue and crack, which makes latch problems even worse.