99 tailgate keeps getting stuck shut

[2talll]

The past couple days the window to the back of my tailgate wouldn't open about half the time. Like yesterday it opened when I put my stuff in after my first job and then again at the 2nd job to take stuff out. WHen I was going home it opened to put stuff in but when I got hom it wouldn't open. I hit the unlock button when I was getting out but when I push the button it was stuck at first and didn't go all the way in but the it just slid all the way in like it does if you push it when the window was already open.

This morning it opened fine. Tonight when I went to put stuff in the window was stuck just like it was yesterday so figured that the window probably just got frozen shut since the sun was out and shining on the window and then it got dark and colder so I just figured some of the snow on the top of the car melted and then refroze around the window. But the entire tailgate was stuck as well. Same thing- I tried puching the latch to open it and it was stuck at first but when I tried again it was all loose like it was laready opened.

Any ideas??