Car on ramps (top end rebuild).
Keep finding water in the rear floor pans after a hard rain.

Sunroof drains - ok
Drains in rear of fenders - cleared
Windshield gasket - needs to be replaced, but not leaking in.
Front/Firewall cowl drain - clear.

Removed wipers, cowl, leafguards, and inspected the
cowl heater area.

Found a 1/4 drain hole just rear of cowl/firewall drain.

Rubber stopper deteriorated and was letting the collected water
drain into the cabin, down the tranny hump, and into the pass
rear seat pan.

Another suggestion I heard is to "reroute" the fresh air drain.

On my car (has a/c) the fresh air drain is only on the drivers side.
(the pass side is plugged w/styrofoam).

Tube drains into space behind front driver's wheel
(where vacuum accumulator is).

That area is usually sealed off by the cover, so water has no place to go.

I suppose you could run the tube through the cover,
but there might be another way...

joe