I can help you....
If it were rainwater getting in then you'd notice it when you drive in the rain. I guess that you do drive in the rain - and I also guess that you only notice the damp when you get in but not when you get out.
So - based on what you say - it's probably not caused by rainwater.
That only leaves condensation - warm wet air is condensing out on the inner surface of the cooler fabric of the roof - which slopes down towards the front and dumps the water on the sun visors.
You do not mention if the roof is also wet on the inside but no matter.
Best perhaps to let the warm air out before parking up.
But all this is pure guesswork..., May 2015