I have a 1971 Ranchero that leaks onto the carpet on the passenger side when it rains. The water drips from under the dash on the far right side. I do not think it is coming from the windshield.
Check the cowl drains on both sides, they emerge on either side down low just froward of the door opening behind the fenders, if they are plugged the water could flow in the fresh air intake for the heater. Another problem is the cowl's interior metal could be rusting out and leaking water right through front bulkhead.
I have the same problem on my 1975, I am going to clean those drains and see if it helps, I will report back. Gary
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