I'm dealing with this on my 88. After I got the two ports opened up and got all the debris cleaned out of the cowl, I could see that it was mostly just the seam that was not sealed well anymore. Since everything in there was still pretty solid and it was just a seam that the sealant had dried up and disintegrated years ago to deal with, I took the hood off, climbed on top of the motor, and reached in with a wire brush and screwdriver to scrape what I could. Since I was tearing the truck down to replace the radiator support and some other things anyway, I took the drivers side fender off and cut a larger hole in for the drain and reached in that way as well (found a giant mouse nest plugging up that side too). Once it was cleaned as best as possible and dry, I got out a tube of Locktite PL-S30 roof sealant and re-sealed the seam. I used a popsicle stick taped to a screwdriver and one taped to a stick to reach the seam and smeared it on there good. Pretty sure the rest of the truck will rust away before that stuff gives up. A word of caution though, the stuff acts like tar when you put it on and it's very slow to cure, but once it's cured, it's there for good.