General Tools MMD4E Digital Moisture Meter, Water Leak Detector, Moisture Tester, Pin Type, Backlit LCD Display With Audible and Visual High-Medium-Low Moisture Content Alerts, Grays

The rugs are a big suspect... moisture meter gun? I never needed might find what part of car rug you might want to cut a piece out of to inspect first....unless you know of a leak area that many cars have suffered. https://www.amazon.com/General-Tools-MMD4E-Moisture-High-Medium-Low/dp/B00275F5O2/ Or from the air vent! Removing the blower fan would be an idea to look for...starting by looking in and around the cabin filter...

Get a few damp rid bucket for boats. https://a.co/d/08uysLKJ You'll need to get everything out and wash it all before it's molding. Same with walls, ceiling and flooring Check those with a moisture meter like this https://a.co/d/07mbiDWz