I got this during the snow, bringing the snow in on my feet. Foot well heaters on while driving fixed it, window slightly ajar.
Got it so bad last year it was setting my alarm off.
Could be from damp mats, I though it might be because I've had the heaters on so inside the car is warm and outside is cold so it causes condensation which then freezes. Is that theory possible?
Don't seem to have any leeks.