I have a 2001 model V6 AUto CDX which I have owned since new, and have loved almost every mile!
However, one problem that I suffered almost since new was that the windows tended to steam up very easily, and were almost impossible to clear using the climate control system. Even a heavy shower in the summer resulted in the windscreen misting up, simply becuase it cooled down the glass.
The only solution I ever found was to open the sunroof or windows to clear the exess humidity, although the problem returned as soon as the sunroof was closed again. At one stage I suspected that water was getting in somewhere, but I disproved that theory.
I know of other people who have suffered simialr problems in other Vauxhalls including Astras and Vectras, alhough my brother-in-laws Omega was never a problem.
Anyhow, after six years the original battery failed, so I fitted a new one myself. As if by magic the aircon problem dissapered, the car smells much fresher, and the aircon itself seems to work much better. As far as I can work out, this was the first time the battery had ever been disconncted, as I have never had to enter the radio code before, and our dealer doesn't know what the code is.
From what I can work out, the climate control system is electronically controlled, and uses firmware to manage the system. I suspect this had somehow become corrupted, therefore disconnecting the battery forced it to reboot.
It seems the system was recycling air regadless of the user settings, and could not be resolved even by pressing the Auto and Off buttons together as reccomneded in the manual.
I hope this helps other readers. And if not, at least it doesen't cost anything to disconnect the battery for five minutes!