Thanks for the suggestions, guys. I checked the cowl drains, and they're clean as a whistle. I went over the car again looking for wet spots and didn't find any. I went ahead and put a giant box of baking soda in the trunk and another in the passenger compartment, just to see if that gets me some temporary relief.

Per another thread, I was finally able to get my back seat cushion out. All kinds of garbage was hiding under there, but nothing that should smell bad and no water or rusty spots.

I'll do some searches about how to replace the cabin air filters -- sounds like that might be a DIY job for me next weekend.

Thanks again --
Cheers, John

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'98 E300TD (W210) | Sold 6/09 with 205k and counting
'04 VW Touareg V10TDI