I had to re-glue mine after I had the whole headliner replaced under warranty once the weather got warm again. I used the clear 3M spray cement.
Just gently peel back the sagging area and give it a good shot of adhesive on the roof of the cabin and carpet as well.Start from the middle and work your way back to the edges. I found when I did this repair I had excess carpet where it met the wall. I had as much as 3/8's to 3/4 of an inch. I just used a brand new razor an trimmed it back very carefully. It has been perfect since. When I get the radar installed next week I am going to hit the center of the carpet through the access hole in the rear an the light which I know has to be taken apart anyway. I didn't do this when I did the repair and it sagged right in the middle. After that I think I will be home free for a long time