Carefully....drill a small hole in the bottom of the lens. You can't stop it getting in but you can show it the way out!
I looked at this option as I had one rusty side which I managed to release, clean up and fit a new fibre washer (which had disintegrated)
I discounted drilling as these lamps are tilted and water would still gather below the lens. I opted for a liberal coating of vasoline inside and spayed with waxoil all around the back
