It looks like I solved the issue, although having changed two things, I can't pinpoint what really did it. Poor scientific method, but when your windows are fogging, you just want it fixed!

I suspect that the gaggle of leaves and dirt surrounding the intake filter under the cowl was the real culprit. The fogging only really occurred when the fan started sucking in air, and the engine warmed up a bit. If the warm engine began heating whatever moisture was trapped under there, a fair bit could have gotten into the cabin that way.

Hopefully this will continue as our rainy season cranks up. We've already gotten 3 inches in the last week.