Is it the rubber strip on the outside of the car, like an inch wide? If so, its not so much a seal, but a trim piece, the windsreen is sealed in by the urethane, (or what ever that black **** is they glue it in with?)
When the windscreens installed, that rubber bit should be on the windscreen and installed in one piece.
The urethane/glue usually squashes around the rubber and makes it just about impossible to pull out/put back in once the screens in place.
If you can see the glue in the gap between the screen and the body/roof of the car, you can try and cut it out with a stanely knife, trying to be careful not to scratch the paint too bad - will lead to rust, or chip the screen,
If that doesn't work, you can cut the 'T' piece off the back of the rubber and glue it in place with the same black urethane glue.