Q: How to stop water from leaking in behind firewall into passenger compartment? on 1994 Pontiac Grand Am

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This began years ago, and has taken me nearly 3 years to determine where the leak is coming in. Car has been sitting since 2009, and needs to be sealed. Apparently there is a great possibility that it is a manufacturer defect in behind the vents outside, beneath the windshield, but GM claims this leaking, running down behind the gas and brake pedals and pooling on the floorboards on both sides of the vehicle is due to car owners leaving the windows down when it rains. In otherwords, GM refuses to acknowledge the likely defect and cause of the leaking problem at all. Would hate to have th junk this great car. Never had any major problems with it otherwise ever. Hope someone can help. Thanks.
(2) Answers
(1) Comments
it is probable heater core if you lived in northern states you would know by steam on windsheld when you use your defroster
Thanks for answering, 1lost, but actually, it only happens when the car's sitting (ie: not running at all,) and it rains. In fact, I think the heater core was replaced around 03 or 02, but I can't recall, and I don't have immediate access to the service record files right now to confirm that it was. I know it was recalled by GM back in 02, I think, for a faulty ignition switch. That was fixed by a dealer, but the water only comes in if it rains or I run water over the hood and windshield with a hose. I'm really trying to figure out how to get to where the water is getting in, to seal it with 3M sealant I got to put at where the firewall meets the top of body, I guess. There seems to be a cowl down under the dash with something that looks a bit like the metal flashing material that's used on roofing. I don't want to have to tear it apart and not be able to put it back together. But thanks for your suggestion about the heater core.
Don't know if this will help because I did it on a 2003 grand am but had somewhat the same problem but along with the water inside it took out several blower resistors and Body Control Module. Both behind the glove box. Under the cowling behind the wipers there is suppose to be a small u-shaped piece of foam which deflects water around an opening which leads into the interior. This foam can go bad and water on the floorboard is the results. It is usually on the passenger side but water has a strange way of going places you wouldn't expect. About 4-5 years ago I was able to order it from a GM dealer but don't know if it is still available; if not you could probably make your own.

Written 10/16/2016