Unfortunately some Alfa 156 owners has experienced water leakage.
If you during rainy weather experience that the carpet gets
wet then you should keep reading. And don't think you might
not have this problem anyway! Because the carpet should be
water resistant on the down side, so you might not know before
looking under it!
Have a look here for a detailed description on how to look: wetalfa
Thanks to Bjarne for this info!
One additional place water can enter the car is by the roof at the end of
the top of the back doors:
There is a space in the metal between the roof and the door which has
been known to cause some leakage.