During the Rainy Season - I found out my '71 Duster had sprung a leak!

The door seals are another point of contention on any of these older hardtops and some sedans too. What I would do would be first test the windshield like outlined.....maybe even tape over the cowl vent to isolate the windshield area. Once you test the windshield, then you can untape the cowl vent and shoot the hose in it to test it out too.