Daz they aren't sealed properly. You need to take out the screws in the back of the headlight housing and put in the oven for 10 mins at 100 degrees centigrade. That will soften the sealant up enough for you to pry the casing open - depending on what type of sealant was use to reseal the headlight.
Once you have it opened you need to dry out the inside with a hairdryer or something but a really good tip I got was to put a couple of those silica gel packs you find in shoe boxes these days inside the headlight housing when you reseal it as they absorb massive amounts of moisture - obviously making sure you put them behind the silver reflector where they can't be seen or are in direct contact with any heat.
You need to reseal using a rubber butyl type sealant which is flexible but don't put too much in the channels or the 2 halves won't push together properly and therefore won't seal properly.
Put headlight back in the oven for 5 mins at 100 degrees, remove from oven, put in the silica gel packs, apply the new sealant, squeeze 2 halves together and put screws back in.
That should be you sorted!
This is the type of stuff to use:http://www.ebay.co.uk/itm/Rubber-Based-Strong-Butyl-Sealant-Caravan-Car-Home-DIY-Glass-Panel-GRP-sealer-/221384238072