New member, new truck, leak under dash

[clockdoc]

Good morning all. I am brand new to this forum and a brand new square body ('78 shortbed) owner. We had a real downpour yesterday and the floor under the dash quickly became wet, as in standing water. The water was dripping from a small (maybe 2.5"), circular hole above the brake pedal. The hole contains a disc, with directional louvers, that is rotatable to direct airflow. This hole appears to be in a heat/ AC duct. Any ideas as to how the water is getting there? Any help will be much appreciated.

Clockdoc