I'm battling it myself and so far I'm keeping it at bay.
I have been using por-15 to help with the rust. What I do is drill a hole in the cab corner and get a syringe (w/o the needle). I then squirt the pro-15 into the inside of the cab corner trying to squirt the stuff all along the inside hoping it will react with the rust like it's suppose to. After that I bondo the hole and spray the corner with the color match paint.
If by chance it gets worse I will keep doing the pro-15 but then I'll move on to using fiberglass bondo to rebuild the corners if need be. Hopefully by then I won't have to worry as I will have another truck.

 

I took care of mine the brutal way: had the body shop replace both rocker panels.

Then I drove to Texas for work and got a clean box with no rust.

Had to replace the AC compressor on that trip.

This was an expensive summer.

 

I had the luck to recently purchase a 2005 screw that ended up having rusted out corners, even down here in Florida! My forensic analysis came to the determination that the third brake light AND the leaking back window frame and slider all contributed. An old man owned the vehicle and used it for work and based on all the rust on the back seat hardware he never noticed a problem. The third brake light had a large hole in it caused by the heat from the bulbs softening the plastic and distorting it. This made a sort of funnel for water to just pour in. All the water has to go somewhere, so it pours down the holes for the body mount/seat belt mounts, and drains into the corners and the foot wells. I wouldn't be surprised if the front leak areas make their way to the cab corners too.

The cab corners are readily available and easy work for a good bodyshop. There is a new ford third brake light that has a better gasket. I wouldn't use anything but this one. Seal the screw heads with clear silicone. There are seals available for the slider, and urethane will seal the frame better than butyl rubber.

 

jesus christ, what design flaws these trucks have. the back window too? mine blew out due the defrost issue (another fail) and had a new one put in, would i still need to seal it? thanks sir.

 

they make cab corners.

 

You're surprised an American made truck that's 15 years old that lives in the rust belt has rusted and the weather strip apparently failed?

They didn't design them to last forever, things wear out.

 

mine is 8 years old and don't act like it is acceptable. i have seen people that start to get rust after 4 years and they don't even have the pos paid off yet. next.

 

my wife has a 08 toyota highlander with 0 rust.