Has the yellow lens from your signals ever been removed for bulb changes or anything else? There are 2 little square notches cut on the bottoms of each lens. I would assume that one purpose for these cuts is to allow water and/or mositure to drain out of the housing. Is is possible that your lens are on upside down? I know that there's a "top" indicator on the lens and they don't fit on the proper housing if upside down, but if the left lens is upside down on the right housing and the right on the left, they will fit and the drain notches will be pointing upwards (basically allowing even more water/mositure into the housing).

JUST A THOUGHT

But yeah, the white rubber gasket missing from between the lens and housing would be the most common cause.