Firewall/Under Dash Water Leaks

What he said...
Plus any of the firewall seals to things like the steering column, brake master cylinder etc.

Water can splash up there or if you wash off the engine compartment it van get in that way.

Best thing to do is get it all dry, have some one with a garden hose work from the lowest point to the highest point with a light stream of water while you (with the front seats removed) lay on your back with a flashlight looking for signs of water.