Wouldnt be too hard to work that. You have two options:
1) Heat up some water so that its almost to a boil. Dip the face of the lens in the water so that the sealant around the edges gets warm and pliable. Alternate between heating and prying until your get the lens to separate from the housing.
2) pop that bad boy into the oven. Start with low temps around 175 and work your way up until your find a temp that melts the sealant, but not the light. Pull the light out every 10-15 mins to try to pry the lens off the housing.
Once you do that, have at it with some windex. Be very careful not to leave streaks, as they certainly will show. Once theyre cleaned, lay down a bead of RTV Silicone ($11 at parts store, they have multiple colors) and carefully lay the lens back down. Make sure you form a good seal, as this will prevent more moisture coming in. The sealant takes 24 hours to set.
Any questions let me know. ive done this on 5 different headlights before with decent results.