How to stop water from leaking in behind firewall into passenger compartment?

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.