My old Nissan would leak water only when it would rain so hard, that the cowl intake drain would be overwhelmed to the point that the water would pour into the heater intake. 

I get some seepage in my 87, and it comes from the rear window gasket, and on the passenger front.  It's not alot, just enough to kinda get you all bent once in a while.  Then I think back to my old El Camino, it was like Niagara falls when it rained from the heater.  And would rain on me from the sunroof.