Water in blower housing after rain; what's wrong?

[xsdg]

Hey, y'all

 

So, I've been using the defroster all summer long, no issues.  First rain today.  I go to defrost the windshield and there's water going every which way from the blower housing.  Truck has been off and stationary since the rain began overnight.

 

So, obviously this isn't supposed to be happening.  Anyone have tips on what to check or where to look?  I looked in the FSM, and it describes all of the HVAC stuff that's inside of the cab, but doesn't mention how exterior air actually gets into the cab, or what pieces are supposed to be in place.

[Draker]

Pull the cowl vent, clean it all out if there is shit in there. 

 

My guess... you've got a rust hole or shit blocking the drain tube causing water to come in.

[datzenmike]

There are drain holes to the extreme right and left of the plenum under the cowl. (where the wiper arms are) The drains let trapped water out behind the front fender and it runs down and out behind the back of the front tire, just forward of the top door hinge. These plug up with pine needles and maple keys. The air inlet to the heater on the 720 is in the extreme right corner. It's raised slightly so water will flow around it to get to the drain. Wouldn't take much to dam it up and over flow into the heater.