These light units are serious money, I was in the Dealership recently and someone parted with £340 for a VW replacement - not sure if a Mk 5 didn't see the guy drive off.
To fix the problem, park vehicle facing the sun, to remove all moisture from the inside of the unit, or take-off and put in the airing cupboard for several days. When you are satisfied that it is REAL DRY. Find the crack, very lightly abraid the surface along the crack with v.fine sand paper, then apply two pack epoxy resin along the crack so that it runs in. Dont use Araldite if the crack is on the lense and you want a cosmetic repair. Use runny resin and hardener. To make sure it sucks into the crack if hair line, apply the epoxy whilst the unit is hot from the sun or airing cupboard, then as the Head light unit cools - air inside the unit will shrink a little and just enough to pull the epoxy into the crack itself.
A permanent and cosmetic fix is achieved if done just right.
The same principle as above, is applied by those mobile windcreen crack repair guys.
Washing headlamp units with a hot steam jet power cleaner is not a clever idea. The rapid change in temperature (particularly in Winter) will bust the manufacturing seals.
