The problem you will have fitting an aftermarket sunroof to your Fiero is because of the way the Fiero's skin is built as a plastic non-structural shell over a structural metal frame. There is already an opening in the metal structure of the roof, it is quite large, so your aftermarket sunroof will be held in place by attachment to the non-structural plastic skin of the roof. That will weaken the skin, as well as not be a secure fitment. If it were me, I would rather do without a sunroof than put in an aftermarket one, or spend the money to get the sunroof parts and roof panel from another source or country.
JazzMan
