Last night my brother reminded of a couple of things my ex-wife did 25 years ago that I'd forgotten about. My dad gave my then wife a bag of peaches one day while she was visiting my parents. She put the bag in the floor behind the passenger's seat and forgot about them. They sat there for days until they turned to mush in the heat and fermented. Her car smelled like peach schnapps for months and the carpet was forever stained.
Around the same time she left two half-gallon containers of ice cream in the trunk during summer. They melted, ran all out into the carpet of her trunk and spoiled. She didn't think about them again until a few days later when the smell became overwhelming. It was stomach-churning. It would've smelled better if she'd driven around for a month with a dead body back there. It took months of cleaning and spraying and lots of Carpet Fresh and cleaner to finally get rid of that smell.
Two kids under four in my house, so we have sippy cups with milk in our cars often.
I make sure to clean up anything the kids bring into my car. My wife, on the other hand, will let things sit in her car for a few days until she realizes we're running low on cups, THEN she'll clean her car out.
A few months ago, a sippy cup got hidden between one of the car seats and the center arm rest/divider in her back seat... the cup was tilted pointing down. So there was a slow drip of fermented milk getting into the seat cushions in her car over a warm weekend. The smell when we opened her car doors on Sunday night to take a drive was horrendous.
I've blasted the back seat, the arm rest, the car seats and seat covers with industrial grade odor eliminator from the body shop, but the smell persists.
My next course of action is to see if I can grab replacement seat cushions and covers for the back seat.
After that, she's SOL and getting a new (cheap shitty) car lol.