Try using a garden hose with a nozzle that you can get a high pressure spray from, either you or a helper get in the car so when you soak the car with the hose you might be able to see where it's coming from. Good luck.
I had a leak and it turned out to be the wiper arm gaskets on my 73 duster, How I found it was to get inside the car, and looking under the dash while having a friend run the water over the car. You could also pull the kick panels when running the water and try to locate as I did.