Where is this interior leak coming from?

[AuroraGirl]

Woahhhh… where did you find this?

Online on a service for service data, but I sent it for it shows just one way you can have a leak. If you have a AC truck, the same leak probably could happen when in... recirc? I forget which way the valve moves. But its more or less about the water deflectors, make sure yours are in position and in good shape, maybe one is missing or not positioned correctly?
There are also hood hinge seals on the 81+ square iirc. Sealing as you suggested the assembly into the opening is definitely wise, I bet it prevents water from finding its way in under pressure while driving or as a drain path while still. If you could find butyl in sheet form, that would make a good sealant too,or if you had it in rope form and worked it around the opening enough to put your vent in.

 

[AuroraGirl]

https://67-72chevytrucks.com/vboard/showthread.php?t=683053 heres some more good info and pics of the possible causes, the cowl area with the wiper transmission and the spots not seen outside of the truck but behind fenders appear to be places to check if you cant stop the leak at the vent assembly with sealant or a gasket (apparently they have gaskets?)