Water leaks can make you go crazy, that's the only certainty. Best way we chase them is, have a person get inside where the leak is, with a flash light, have another person start with the water hose. Start low, and work the way up with the hose stream.
It's not so bad in a cab, usually end up laying in the floor board or being upside down. Inside a trunk though, sucks for the little guy in the shop, which ain't me! I chased a water leak on the firewall in an el camino once, I ended up using sprayable seam sealer and just coated the dang firewall inside and out, that fixed it!
