Unfortunately, the Grand Prixs speakers, on the outer ends of the dash, are integral to the dash and you have to put the speaker in on the under side of the dash which requires that you almost have to fully remove the dash to get to them. The good news is that you can get or create a spacer to accomodate the higher domed tweeters. Best to find a pontiac with this style dash in the junkyard to experiment on.
Or on some cars you can use the center speaker area, but that will forgo having true stereo.
I like these pontiac dash designs the best overall, with the exception of the speaker placement design being the worst of the bunch, since it is almost a must to replace the original speakers.
Good Luck.