I also purchased CPO and inherited this problem. I checked the drain plugs and everything seem to be ok and dry in the car. But the smell was bad after the rain.
This past 4 months I’ve used 2 different cleaners Nextzett and Kool it and it works for a few weeks and the smell comes back again...
Thank you for posting this. I’m in California and it rain last week. I’m going through the same thing. My vents smell really bad and the can of Lysol didn’t help at all.