Yeah, my 05 4x4 does this too, normally just the passenger side and only really decided to do it after the warranty expired. Water seems to run down the 'gutter' on the inside of the front wing and gets tipped onto the top of the light unit just where the cover is to change the bulbs... I smeared the seal on that cover with silicone grease, which seems to have helped keep the water out but still lets the covers pull away easily to change the bulbs.
P