Carpets are wet in foot wells after all the frosty weather, and not just when it rains. Any solutions?

How wet is the foot well? This is likely to be one of two things. Door seal rubber or the Heater matrix.
Try this bit of detective work. Does it only get wet when it rains heavily? Is it just the front that is wet? Feel the carpet in the foot well. How high up the foot well carpet does the wetness go? Does the wetness smell of anything? Do you have a sun roof fitted? Sun roofs have drain holes that can clog up so the rain pools up in the grooves and runners then over spills inside the vehicle.
If it only gets wet after rain it could be a worn out door seal rubber or the sun roof letting water in. Door seals usually cause shallow pools of water so the wetness won't rise very high up the carpet. The sun roof sometimes causes the seats to get wet as well.
If it is wet all the time, the wetness rises high up into the foot well and it has a 'sweet' smell, it could be the small radiator unit,(the Heater Matrix), behind the dashboard that supplies heat to the heater blowers leaking. Depending on where exactly it is fitted you'll sometimes get wetness in both front foot wells. The smell is the coolant from the engine.
Hope this helps.

Check the windshield seal and clean the front right under the windshield. There are usually drain holes for rain water to drain away. If you are getting wet without rain, your AC drip pan drain is probably plugged and needs to be cleared out.

The two most common causes are rain water leaking in or the ac drain being clogged up. I would vote for rain water if the entire carpet area is wet.
Could be coming in around the windshield, or one of the door gaskets.

DRAIN HOLES IN BOTTOM OF DOORS? UNDER BONNET BELOW WINDSCREEN CAN COLLECT A LOT OF STUFF

I would assume they got blocked and have rotted, thus allowing water into foot wells

the heater core is leaking

Check under the carpets there is often a drain plug in the floor well which may be missing. The other thing that can cause this is if you are using the aircon and the drainpipe has disconnected from the evaporator drain pan. this is a pipe that runs from the drainpan through the firewall (cab and engine wall) to the engine compartment.

You need to check the trunk to see if it is closing properly, the other thing will be the weather stripping around the right rear door

Most of the problems come when its raining and if its not being driven and water still comes in then its the bonnet rubber and if when its being driven then its due to the rear wheel throughing up water under wheelarch and somewhere theres a crack in the weld or somewhere and it will find its way in..
So if not using it clean it out and when rain stops look inside no rain no leak Have a goodday

Trace which way the water went down ,,, a major plug around one of the front foot wells could be full of water ... also now you need to check all the fuses. Push on the carpets front and rear floor to see if it's not flooded out under the carpets. Some cars have computer modules under the front seats.