Hello and welcome to JA!!!! the first thing to check, there is a weatherstrip just under the cowl area. right where the right wiper arm sets. this weatherstrip is supposed to sit on the bottom of the glass. but they will fall off and cause water to run in the a/c box, and then in the vehicle. here is a picture. the arrow in the picture shows the weatherstrip seal
ok, there is a waterdeflector that gm makes, that will deflect the water away from the cowl area. this part was orginally made for monte carlos and grand prix's. but wil work on impala's. i have used this deflector before and normally takes care of the leak. it is easy to install and is cheap to buy.
Install the new designed plenum water deflector.
Install the air inlet grille panel.
Install the push pins.
Install the washer hose.
Install the hood seal.
Close the hood.
Return the windshield wipers to the park position.
should be, maybe a part number change.
#03-08-57-006B: Water Leak on Front Passenger Floor (Install Water Deflector) - (Feb 25, 2005)
Parts are expected to be available from GMSPO on 3/2/05.