Greetings! Now where is this rust coming from?

[sixtwo]

Hello, all
BACK STORY:
New here, just picked up a 70 c10...long bed, auto, 307, nothing special....
Not my first classic, but my first living on the West coast.
She's not pretty. Dent in the bed and cancer/ rot on the doors, rockers, floor pans and kick panels. Actually not that terrible but I was still going to walk away from it. Has new tires, exhaust and shocks all around.
Was about to walk away when the seller said he's take $300 for it. How could I say no? The abundant yellow jackets came free with the truck too!

Anyway, ripped out the rubber mat flooring and say a lot of moisture and rust. Looks like it comes down the inner kick panels and onto the floor which rotted out my rockers and lower doors.
Question is, where is the water coming from. Seems to be a common issue. Is it from bad window sealing? Or does it run down the cowl some how?
I want to fix the cause before I start on the result of the cause.
Thanks for any help!


This is the photo from the ad where I picked it up