Permatex Flowable Silicone (item # 81730) is a low-viscosity silicone sealer that “seeks the leak.” It flows easily into little cracks and crevices, and, after a 24-hour cure, forms a clear waterproof seal. Typical applications include windshields, sunroofs, windows, headlight assemblies, RV vents, and marine glass.
I found and used it a few years ago to successfully seal a small, but annoying and persistent, leak at the top of the windshield in my trusty old pickup after an ever-so-helpful dealer said the windshield would have to be removed and a new gasket installed. I have also read about its use in sealing outdoor PVC active antenna assemblies for amateur radio.
1.5 fl. oz. tubes are available online for about $3.
By the way — as you might have guessed from the tag line on their logo — Permatex® celebrated their 100th anniversary this year.