How is water getting into my cab?

[Gumby83]

I have a very rusty old farmtruck i am fixing up, i thought i got the cab sealed up well. but this is the first time i've had the truck out in the weather since and now i notice i have two wet spots now. the PO had to replace the drivers floorpan so maybe we both never caught where this water was coming from? any help would be appreciated.

[Braunschweiger]

Look under the dash for the Cool Vents channel.
Other places can be windshield and roof gutter
if there is no seal left.