It is common for those lenses to dry and micro-crack, allowing water to sneak by and into the cap interior. Although there may be a provided gasket, many of us ensure things by adding sealant anywhere water might find entry.
I and others here have come to prefer a product called
Lexel in lieu of silicone sealants. It can be used for any sealing jobs. I've come to use it at home as well, anytime I drill a hole for screws, etc., in an outside surface or apply in some way something where I want to be sure rain or sprinkler water can't get past. You can work it with a wet finger similarly to silicone products, but it is not silicone, and cures relatively fast, so should be worked soon after or during application. I use clear for everything, but there is a white version.
My only complaint is that if not carefully capped, it will plug its nozzle during long storage, and I must squeeze out a couple inches from a stored tube before getting to less viscous, workable product. Trying to use thickened stuff is a pain. But fresh Lexel works easily enough. And properly, cleanly applied, it seals exceedingly well. It can be pretty sticky and can even be used on wet surfaces, so is handy in emergency situations such as leaks during rain. 10-14 years ago a badly deteriorated upper windshield escutcheon and brow was feeding rainwater inside our cap and filling the electric bay. I used Lexel to fill all the cracks and gaps in the rubber, and smeared thin clear layers over the yellow clearance lamp covers that I knew could be a problem, and not a drop has entered the cap or bay since.
Lexel is available in handy squeeze tubes and I've seen caulking gun sized ones at Lowe's and other places. Ace hardware has the squeeze tubes hanging over their caulking tube display.
Contrary to the Ace description, it's not "rubber".
https://www.acehardware.com/departments/paint-and-supplies/caulk-and-sealants/caulk/17531https://www.sashco.com/products/lexel/Joel
