97 Grand prix leaks water after rain in passenger side floorboard

Does anybody know how to fix this?

Check the condition of the rubber weatherstripping under the plastic cowl passenger side which is the air intake for the HVAC system. Also, there is a TSB applicable to your year Grand Prix which covers a fix. Another thing you can do is add a cabin air filter if you do not have one already installed. '97-'03 Grand Prixs do not come with one from the factory. You can use one from a '97-'04 Buick Regal.

Last make sure the HVAC drain at the firewall is not clogged.

A cabin air filter will help reduce or eliminate water intrusion.