This is another common Dodge truck problem. Have had to repair a few of these. The rain gutter is attached to a flanged seam where the roof and window frame are spot welded together, then sealed with plenty of seam sealer. This is a common area to find serious rust problems, if you find rust here check the area under the rain gutter covered by the windshield rubber. Also check inside above the seal as well, my 79 was badly rusted there. I have my current truck oil sprayed every fall, and so far don't have any rust. The company that sprays my truck, Krown Rust, sprays under all the trim and around the window seals. They use a product that displaces water, it is also used in aviation aplications. It's a bit messy but worth it.
Neil
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Neil Copeman 92 D250,LE,limited slip, 16cm turbo housing, Isspro gauges, factory tach option, Lear cap, DC crankdown tire carrier
Hadley air horns, Stealhorse leather seats, e-mail (home): <bombedcummins@sympatico. ca> (link at top is work adress)