Buy de-humidifying “moisture bags”. When you sleep in a car and it’s cold, the moisture in your breath fogs the windows up and can cause major mold issues really fast. Also lining the windows with mylar emergency blankets can keep it a lot warmer inside.
I have an anxious fluff with the finicky drinking habits and the feline version of interstitial cystitis, so I’m well acquainted with this brand of crazy. He is very particular about how his water is served, and I’ll do pretty much anything to keep his bladder in working order. His favorite lowball glass on the bathroom counter was looking water-spotty, so I swapped it with a different glass to descale it. The new glass ended up in the sink. Twice. He was considerate enough not to push it off the counter to shatter, but I digress.
It might seem counterproductive, but I’d start by dousing the area with an enzymatic cleaner. I like BacOut, because it’s less punch-in-the-face citrusy than other brands, but any enzyme cleaner would likely help. I’ve also had good luck with FunkAway for mildewy odors. Give it at least 24 hours to do its thing.
Next, I’d use a product like Damp Rid to pull moisture from the carpet. In this case I’d probably use the packet type meant for linen/clothing storage, so you can lay them directly on the floor, instead of the canister style that you can stash in closets and basements. I don’t know how mischievous your cat is, but if he’s the curious type, maybe place the packets in an old pillowcase so he can’t rip one open and get at the silica. These or similar would do the trick nicely. If you have a portable fan, maybe position it nearby to help with the airing process.
THEN, once the must has been busted and the area is dry, I’d get a smallish service mat (the silicon kind used by bartenders and baristas, you can find one on amazon for under $20) to place under his lordship’s water goblet. And, depending on how plush your carpet is, it might help to place a ceramic tile under the mat, for stability. A cute acrylic tray would also catch potential spills, but the bar mat can be thrown in the dishwasher and sanitized.
Whew. That turned into a novel. Hopefully some of the info is useful. Best of luck, fellow cat person!