common fault, not with just seats, but most makes are affected.
it could be a leak in the headlight, or i have found that in 99% of cases its simply a lack of air circulation. you can get rid of the condensation by leaving the back of the headlight assembly off for a while.
It usually happens after washing the car as the air is saturated, (humidity) and that you cool the surface of the headlight. Colder air cannot hold as much moisture as warm air. When the air is cooled by a cold surface, say a headlight thats had cold water over it, or your bathroom mirror etc then air cools and if it passes it dew point ( point at which its temperature is too low to maintain the moisture level then it condenses on the surface.
So its very common to happen when washing a car.
Solutions?
1 would be to have sealed head light units, - unfortunately this is not easily achievable - for example if its 100% sealed how do you change a bulb! and also if it was sealed then it would need to be sealed in low humidy levels. If it did get consensation, then it would be difficult to remove.
2 increase air circulation.- eg remove back of headlight and leave off for a while to get good air circulation. (id recommend you do this when the weather is dry).