I am sure that it has to do with my unfixed windshield leaks, which results in wet carpets which makes it more humid in the car, but my defrosters in my 245 just don't quite do the job when the temps dip below 25 degrees. Would the hottest thermostat solve the problem, resulting in hotter air at the vents?

Make sure the recirc door closes properly (button out, to draw outside air), and if you have it, run the a/c compressor to dehumidify.

The heater core isn't leaking a bit, is it?

You should look just in front of the heater core(remove the passenger kick panel) to check on the recirc door; I'd bet it's stuck in the "recirc" position and no outside air is coming in. You may have to wedge it.

A leaking windshield, you say? Tough to fix. In your shoes I would pull the from carpet and mats, and replace them with old newspapers. A lot of them, and put in with a mind to replacing them easily. If you have floor mats or those nice floor mud pans put them on top of the papers. Even use some old carpet (house type) on top of the papers. Just get the water out.

I suppose that below the wet papers will freeze together, so have that in mind when placing them.

Can you get a big plastic drop cloth (get the thickest one they have) from a local hardware store? And a bunch of small magnets? You could cover the windshield, top to bottom and side to side, at least for overnight and while parked at work.

How is your a/c? We have experienced very dry air by running both the heater and the a/c simultaneously. It takes some tweaking of setting the heat, fan speed, and a/c level, but man is it able to dry the air.

Good Luck,

Bob

:>)