You should not assume compatibility. The best way to know for sure is to:
* Check part numbers: Find the part numbers for the doors on both cars. Identical part numbers strongly suggest interchangeability.
* Compare the door shells visually: If possible, compare the doors closely. Look for differences in shape, especially around the window and handles. Even small differences can make installation difficult or impossible.
Simply because they're both second-gen Grand Prix's doesn't guarantee the doors will be a direct swap. Proceed with caution and verify compatibility before purchase or installation.