Best solution for a suspected windshield leak

I have a white (paint is peeling off in sheets) 1994 GMC Sierra. The truck has developed a slow leak just to the left of the rear view mirror and my headliner is damp. I'm worried about getting rust on the inside of the seam so what would be the best fix? Can I try to reseal with some permatex or should I bring it to a glass shop to replace the entire windshield?