[beautimus]

you may have noticed that there are several layers of metal coming together in that area. gm used some really crappy seam sealer to bury the seams. and since its shaped like a trough in that area, it puddles water and when the old sealer cracks, it rusts through and drips inside the cab. i have seen cabs rust so badly in the channel, that, after everything is unbolted you can actually pull the fire wall away from the cowl with your hands. yours doesn't look that bad to me. mine didn't have any rust there but the sealer was in bad shape, so i dug it out before i had the cab blasted. then i sealed it up in fiberglass all the way across to prevent the rust out later.
another place to watch for is the seams inside the cowl. if you remove the cowl cover look to the sides of the cab inside the cowl. those can leak and rust through there also.
i guess what i am saying is check it out better when you get it all apart and wherever you find sealer, dig it out, fix the rust and reseal everything. and occaisonally check the sealer.