Hi Ken. The fogging means there's water inside. It happens when the lamp case (or the dash dial) gets warm (sunshine, warm weather or a hot bulb) and causes the moisture to evaporate. When the temperature drops, the vapour condenses as a fog on the colder parts. LEDs certainly run cooler than filament bulbs, so in a situation like an interior dial will reduce the evaporation step. Basically, the moisture more easily stays as liquid. For the sidelights, direct sunlight is an ever present sourch of heat, so the fogging cycle is more obvious. So the answer for both sidelights and dials is to prevent water getting in.
TC
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