water leak

Get some on inside with a flash light.

Get the garden hose on low and start low on the body

Go to every point of entry working your way up.

ALSO

Look under the car for body plugs missing or dislodged.
Brand new 2006 Dodge minivan would get water under the carpet after a carwash. Turned out to be a missing body plug.

Question...
Was car being driven when rainstorm happened?
Was car on slope when rainstorm happened? If so which direction?

Rust?

Windows / seals replaced?