My second 1965 Dodge Dart.

Ok, I have come up with a plan. I am first going to use abrasive cloth / sandpaper on the underside where you see the pics, and get it down to bare metal. Then apply water on top, and see if I can see where it comes in. After that I am going to let it dry up, and then use duct tape to cover the hole from the underside. Push it hard towards the sheetmetal, and then I will try to use soap / engine cleaner / whatever comes to my mind / something that works. I have already used a hose on my air gun and blown in there. The leaves, and soil, and whatever else that was stuck in there flew around. The inside of the car looks horrible. Have to vacuum inside, but that is a minor issue. Working inside the cowl is the struggle. All 4 drain holes in the cowl are now fully open. So the water will come out there, and I hope to get to the point that the water comes out a little brown and dirty. In other words that the water brings out the dirt in there. If I can manage to get it really clean inside, I am planning to attempt to block the drain holes in there and get evapo-rust in. and then flush with plenty of water, and set a fan heater in the car to get it warm and get the water to dry out as fast as possible.
If that proves impossible or not advisable, I have a second option, but it also requires that I get the sheet metal clean with soap or whatever. I have a quart of Owatrol penetrating oil, and if I can get it completely dry in there, the Owatrol oil will seal and stop the rust, it is the best I have ever come across in my life. And I have tried most. It dries in a week time to a rock hard glazing, and I can either leave it like that, or I can spray some paint in there on top of it.