There are two drains - the one mentioned in that thread is the windshield drain that opens just over the gas tank/fuel pump.
The other one is the condensate drain that is a small tube between the heater box and the floor inside the car. Usually hidden by the carpet. You can heat the tube and then pull it down and out of the way, at that point you can gently snake the drain hole and blow compressed air into the drain from outside the car which will clear the elbow between the body and frame. This will also blow a blob of crap upwards in the car so be sure to cover the opening with a rag or you'll end up with mud in the interior. It won't be much, but it doesn't take much to plug the elbow.
Also check the cowl below the glass. If any of the screws and rubber grommets are missing, it leaves a 1/8" hole into the car and rainwater will find it. Also be sure the hole where the wiper hose comes through is tight.

