Apache.....Here's a product favoured by us 'Brits' for rustproofing the cavities in your cab.
Waxoyl Trust me,coming from the west coast of Scotland I'm familiar with rust!
I've used it on quite a few cars with good results.
It's used in a similar way to Woody's suggestion.
Stinks a bit for a few days after application but the smell soon goes away.
I'm not sure if you could find a stockist closer to home.
