This is going to sound completely daft but its really sensible!
Do you park anywhere under trees, near falling leaves etc, or sometimes have leaves by your windscreen wipers?...I digged out a whole load that had worked there way under the plastic scuttle and gone inbetween the windscreen and firewall.
Anything in there holds the water/damp like a sponge, id say that is probably your cause.
You can check by lifting up the rubber seal on the firewall and having a quick look, if you do have lots of crap in there, then you will need to remove the windscreen wipers and the plastic scuttle and dish out all the crap.