Tired of flooding passenger side

I have a 98 Pontiac Grand Prix GTP. This doesnt happen every time it rains, but every few times it rains my passenger side holds about a half inch of water in it. It is extremely frustrating and the only thing I could possibly come up with that would cause it is the fact that there is no cabin air filter. Does anyone know where the water leaks in from or what could cause this? I didn't have time before work to suck it up because I didn't know until I got in my car to leave, so right now my car is going to be holding this water for about 10 hours. Any advice would help as this is really irritating.

Not having a cabin air filter is one cause. Most of the time it is bad sealing at the passenger side intake cowling. The water is coming right into the air intake above the blower motor and dripping down the inner firewall. Also, there is a drain hose that gets clogged which is preventing water from being diverted to the outside.

Any idea what type of cabin filter would fit into the slot on the 98 GTP?

Any idea what type of cabin filter would fit into the slot on the 98 GTP?

Any brand like AC Delco, Fram, Purolator that will fit a '97 - '04 Buick Regal.

I have a 97 and mine does the same thing. I wish I could find this mysterious drain hose. I haven't been able to find one. One thing you do to make sure that water doesn't come in while it is parked is to make sure you turn it off defrost before you shut off the car. Otherwise the air dam is wide open and the water runs right in. I just recently had to change my blower motor and now it starting all over again.

On rare occasion, in hard rain I'd get water on the front passenger
side carpet. Inspection (by removing the glove box door) showed water
dripping out of the heater box.

I finally traced it to a U cross-sectioned rubber deflector under the
cowling (at the base of the windshield). To inspect and see if this
is the cause of your problem; pull off the windshield-wiper arms,
remove a couple of plastic screw & pull the large plastic
cowling-cover off the car. You will note a 2 foot long strip of what
looks like U shaped weather-stripping at the base of the windshield on
the passenger side. This is supposed to prevent a deluge of water
from coming off the windshield & poring into the heater box. Mine was
hanging loose.

The same thing happened to my 99 grand prix. Everytime it rains now I get a pretty good amount of rain on my passenger side which comes through my blower motor. I already had to replace the blower motor once because before I didn't realize it was leaking and it eventually seized up the old one. Anyways, ml_nelson you are exactly right!! I finally traced my problem to the same thing you mentioned. I lifted up the windshield cowling after removing both wipers and those plastic screws and found that same rubber seal/deflector just laying down in there when it should be glued up along the base of the windshield to deflect the water to the left and right of the heater box. Pretty easy fix, prolly only took me 30 minutes or so to take it all apart, glue the piece back into place and put it all back together. Must be a problem though with the 97-2003 grand prix's because I've heard a few people having this issue now. I think the original glue that holds it up there eventually just gives away from all the bumps, heat, cold, etc that occurs over time. So good luck and hope this helps any fellow grand prix owners!!

Same problem in my 2001 GT. I'm not handy enough to try any of this out, but at least I can tell the shop. Seems like there should be a recall of some sort if it's happening this much. I remember a similar thread about this a few months ago.

I had a 99 GTP with the same problem, and it was the EXACT thing
MLNelson says. Do what he says and your leak will stop.