There are so many water ingress routes its not even funny... As you have a sunroof I'd check that the drain tubes are clear (had this on a Xantia and it made under the back seat wet). The sunroof almost certainly isn't watertight, and I believe its not intended to be. But any crud inthe drains will make it leak.
Next go round the door seals to make sure they are actually sealing and not split etc. Then have the door cards off - my 156 SW has a leak in a rear door, where the water gets in via the speaker wiring hole in the door liner. Drips onto the inside of the seal, hey presto damp rear footwell and steamy passenger side windows.
Another option may be to change the aircon filter - I've seen elsewhere here that when old they can cause condensation, although I don't know why that would be.
Others will just say that the car will get steamy on a wet day with the heating on as damp air will come through the heater and be warmed, condensing again on the relatively cold windows.