have a 87 with low mileage on it, after a rain water is coming out from under the heater core area on to the passenger floorboard. Took the passenger fender off cleaned out very little dirt that was in back of a little dam on the cowl. Cleaned out 2 vents near the a/c lines (in back of plastic fender lining).Still have the leak,any suggestions ?

 

Cover the cowl so water can't get into it
Then run water on the windshield

If water starts to come in to cab then the trough around the windshield has rusted out
It is fairly common on older vehicles, the corners have water sitting in them most of the time and it rusts out.

Windshield sits on a raised ridge and it is sealed to that ridge.
All the way around that ridge is a trough that is covered by the window trim
You can remove this trim to inspect the trough
Be VERY CAREFUL using any metal tools at the edge of the windshield, it is tempered glass and will shatter if pressure is put on its edge

Windshield should be removed if you need to repair the trough