Here is my story:
I tried something like this on my 1986 Ford Bronco. I found a product at the local auto parts store. It was esentially a flat carbon filter that was waterproof (about 12 inch by 6 inch). It was sold as just that..a filter for the incoming air in your car.
It was meant to go under those vents in between the wiper blade arm shafts (below the windshield). I removed this metal piece (which was a pain in the a$$ to remove), layed the filters on there, and put the piece back.
I used the A/C all during the summer. The filters worked OK. However, a big problem developed in the winter. When it would rain, the rain would get trapped in these filters. Especially if it was during the winter and often cold, the water would never dry out of the filters. This caused them to develop a nasty mold odor. I quickly removed them and never did that again.