The good news is that, if the smell only comes when you turn the rear on, you almost certainly have something in the rear A/C plumbing rather than something more expensive like a large coolant leak or a failing heater core. Unfortunately, if there's good news, there's often bad news. My guess is the "something" in the rear is almost certainly a mouse nest (or 10). The reason I say that is, old french fry, a dropped orange peel, half a muffin or even a whole cheeseburger (all things my kids have dropped on my van floor) would have dried out and desiccated to the point that nothing could grow on it anymore. Mice, however, make a nice fur and fluff clump which sucks moisture out of the air and re-hydrates all their urine.I'm guessing the previous owner didn't use the rear A/C or heat much, because when I turn it on there is an acute mildew smell in the cabin that is otherwise never present.
Any tips on how to take care of this?