I had a similar leak on my car,I found rust under the battery and in the hinge pocket from plugged up drains.You have to pull the battery tray out to get at the seam between the firewall and inner fender. If rust isn't the problem it could be the windshield seal (my rear window leaks
)or just a plugged drain.Check the hinge area and look inside the fender at the back you should see a tube running part way down ,try poking a wire up it to clear it.Hope this helps.
