TS,
Long story cut short, ur lamps r leaking allowing moisture to enter eventhough it wasnt under d rain.

If ur a DIY kaki, u'll need
- hairdrier/heat gun
- weather seal silicone
- screwdriver

1. Remove d lamp from d car
2. Heat up d joints
3. Carefully pry open d lens from d rear cover. N like Naughty-Kidz said, re-heat d areas tat u hv trouble removing
4. Clean/Remove d ol seals, apply new sealent n reinstall

Appling new sealent on top of d joint might not b as efficient n if ur not careful will leave gaps which will eventually hv moss growing in it.