The 1997-2002 Porsche Boxster convertible tops were offered with plastic rear windows. You can purchase a replacement top with either an original style plastic window or a glass window similar to the 2003 and newer Boxsters.
The Boxster glass window from Auto Tops Direct is a DOT approved aftermarket window with the safety emblems and includes embedded defroster wiring and a wire harness kit to connect it to the vehicle's electrical system. Like the newer Boxster, the glass windows for the 1997-2002 model years are smaller than the plastic window.
This is necessary since the larger plastic window folds when it is retracted into the top boot. Glass window size is 35.5" wide by 12'' tall to allow it to fit into the boot properly.
The trade-off is a slight loss in visibility because of the size and a big gain in clarity in your rear view mirror looking through glass compared to a plastic window that will discolor over time.