Pontiac Grand Prix GTP '99 A/C Filter

Do you mean the cabin air filter? Your Grand Prix was not equipped with one from the factory. BUT. You can buy the filter for the Buick Regal at an auto parts store and it will fit on the Grand Prix. The filter location is under the cowl beneath your windshield wipers. There is a large plastic screw on the passenger side of this cowl plainly visible from the outside. You need to remove the weatherstripping under the hood, this screw and the entire cowl (you may have to be a bit violent, but it will all come off). You'll see the open hole where the filter goes right there.