What can cause a water leak under driver's side dash of a 2000 Ford Expedition?

Answer 1

Bad seal around your windshild or a faulty windshild install. Had a similar problem that shorted out my fuse box and gem module. Windshild was leaking on the top and following the seal around to the bottom where it ran down into the electronic components.

Another cause for a water leak could be A/C related. There is a hose that drains condensation from the interior A/C coil. The coil has a drain pan under it much like the drain pan on a refrigerator. The pan has a hose that goes thru the firewall so the condensation drains onto the ground. If this hose gets plugged, the pan overflows & the water drains onto the floorboard. If this is the case, you'll notice that the problem gets worse as the humidity goes up.