Shalagh, I had the same poblem on my new van [A stone crack on the outside .The solution is Go to your local model shop and purchase the smallest drill they stock ,at the same time buy some clear solvent cement .Drill 2 holes at the end of the cracks[this stops the crack spreading ,then carefully apply the solvent cement over the crack.[make sure it does not run past the crack]
Then from your local car accessory shop buy a headlight repair kit and cut the clear film to a size that just covers the crack ,round the corners this helps to prevent curling.Buy a sticker that will cover the repair patch [cross of St George,CC badge etc etc ] whatever turns you on .
Then if the crack is on the inside ,same again .
If there is condensation inside the unit .You will find 2 small plugs on the edge of the unit ,wait for a hot dry day and remove the 2 plugs for 2 hours ,preferably with the window facing the full sun .Then re fix the pugs .
I did this to mine and it was a complete success. The cost about