I'm guessing then that a sunroof would add about 13 lb of weight to the car.The reason is that if the parts of it that you weighed weigh about 21 lb,then you would have to figure in some weight for the steel panel and part of the headliner that it replaced. A steel panel of that size and whatever headliner is under it that were replaced by the sunroof would have to weigh at least 7 lb or so, and the headliner would add a little bit more,so the net weight added would be around 13 lb.
To figure this I looked up what a common gauge of automotive sheet metal is, which is 20 gauge. This weighs 1.5 lb per square foot.Our sunroofs are about five square feet, so the metal which is no longer there in a car with a sunroof would have weighed about 7.5 lb.