Windows fog after rain - airbox not draining?

[DG Boxster]

My 1998 Boxster has gone from "toy car" in Iowa to "daily driver" in Albuquerque NM. So far so good, except for the lack of power due to altitude.

I have noticed one thing though. I normally use the climate control system in "Auto" but turn off the A/C compressor to save wear and gas. That means I get a mix of outside and heated air. Works fine for me in this season. In summer the top is down so I shut it off.

Now, with the top up, I'm happy, warm and comfy, except for the day after a significant rain (which fortunately doesn't happen too often here). After it rains, I can smell the humidity in the air blowing from the vents, and the windows fog quickly and heavily.

If I turn on the A/C or the defrost (which runs the A/C) the windows clear. This lasts about a day and then I'm back to nice dry air.

I suspect the airbox or something is filling with water. Can anyone suggest a fix? I don't want to tear the car apart but I'd love to have this resolved.

Thanks!