It generally happens because the screws holding the access plate on the back off the headlamp rust into the inserts in the plastic moulding. Then some bright tech comes along to change a bulb and has to break the housing to remove the back cover.
This then stops the unit from sealing against moisture which is the actual problem.
Lots of GTA, especially 147 owners have had this issue and there are reams of info and threads about how to fix cheaply in the GTA lounge.
Suffice to say on my car I bought 2 Ford Mondeo Ballast packs off ebay for £60 each and they worked perfectly, the only difference between ballast packs it seems is the logo on the sticker on the casing and the price Alfa Romeo want for one (approx £200 EACH)
I think I still have the old ballast packs off my old car that were filled with water so can post up some photos later off what you should be looking for on ebay. I'm fairly certain they'll be the same on the GT as it's a derivative of the same platform.
I;d also advise replacing any broken inserts with brass threaded injection moulding inserts along the lines of these
with some good quality button hex headed stainless screws.
The ones I used were knurled on the outside instead of having a self tapping thread.
Hope this helps.