If there is "moisture" as in "standing water," the seal has broken and is letting rain, etc., in. I had the same situation in one of my tail lights. To solve the problem, I went to the back side of the light (i.e. the truck side where you change the bulbs), and drilled a small weep hole though the lens so the water could escape. I used a 1/8" bit and had the hole exit beneath the brake light and above the signal light, so it does not show (of course where you need the hole depends on where your leak is -- duh . . .). Anyway, that solved the problem for me, without my spending a mint on a new tail light assembly. Just be careful when you drill to avoid cracking the lens.