headliner

If it is a fabric headliner which is what it sounds like, you want the wrinkles out...don't use a heat gun. You want to shrink the whole thing so it gets pulled tight on the barbs. Get a kettle of water and put it inside the car and do up all the windows this will do the whole liner. If your happy with the job and only a few small wrinkles are showing then hit with the heat gun, do around the outside of the wrinkle first, in a circle, then slowly move into the center of the circle, heating the wrinkle itself. By the time you get to the center, the outside is cooling and shrinking, pulling out the heated wrinkle in the center.
AL