99 Grand Prix. Water On Passenger Floor

The cowl grate can handle all the water you can give it. There are drains on the ends that drain into the wheel well. The A/C drain is a small elbow on the front of dash. You have to be under the car to see it. Most leaks in that area are from the windshield. The windshield is held in by a

1/2 in. bead of black sealant. Some times when it is installed, a bubble in the sealant creates an air gap or the installers touch something and drag the sealant out of position. Best thing to do is take it to a glass shop and ask them about it. The odd possibility is a gap in a sheet metal seam that didn't get the wipe on sealer at production. Rust holes will leak but it shouldn't be a problem on a '99.

I second the windshield leaking. If the sealent got damaged (as was suggested earlier) or the sealent has dried out (more common on older cars) the windshield will leak like an open window. Don't be surprised if you also find rust behind the seal (and lots of it). I've had this happen too a couple vehicles I have had. My current car (89 caprice wagon - square back model) has one of the fixed rear side windows doing this.

Snow...

I just got back from the Pontiac dealer. One of the service managers said rain water coming down the windshield past the wiper cowl and into the heater box is a known problem. Their solution is to fabricate a water deflector out of plastic, under the wiper cowl and over the heater box. They have done it quite a few times. I will make one and post the results.

Its not glued to the windshield in a '99 GP. It rests against the windshield on a thin strip of weather stripping underneath the wiper cowl. Another option was to pull off the weather stripping and put a bead of silicone where the weatherstripping was. The service manager says the water deflector over the heater box works better. Stay tuned.

A GP I don't know about, but the cowl on a Grand Am is glued onto the windshield. Water is supposed to go over the cowl and not under it. If water goes under it the water goes directly into the blower air intake.

On my G/A the cowl was stuck to a thin strip of weather strip which was stuck to the windshield. When it became unstuck, then water got in. I siliconed it to the windshield with black silicone and solved the problem. Different car though.