Humidity does not just fog your windows. It feeds mold on the AC evaporator and in damp carpet padding, and the smell it leaves behind rarely clears up on its own.
A new musty smell in a car almost never means the car is dirty. It means something inside it is staying wet longer than it should, and in South Florida, with outdoor humidity sitting in the 70 to 90 percent range for most of the year, there is no shortage of places for moisture to hide. The smell is usually the first sign of mold or bacteria growing somewhere you cannot see, not something a wipe-down of the dash or a shot of air freshener will solve.Where the smell actually startsEvery time the air conditioning runs, the evaporator core pulls humidity out of the cabin air and that water condenses on the coil's fins. On a healthy system, most of it drains out through a tube under the car. Some of it stays behind as a thin film on the coil, along with whatever dust and pollen got trapped in the fins first. In a dry climate that film evaporates between drives. In South Florida it often does not, especially on a car that sits for a few days between uses, and that constant dampness is exactly what mold needs to take hold.The smell tends to show up strongest in the first few seconds after the AC kicks on, then fade as dry air starts moving. That pattern is a good clue the evaporator itself is the source, rather than something spilled or growing in the carpet.The cabin air filter is worth checking firstThe cabin air filter sits directly in the airstream heading to the evaporator, and in a humid climate it holds onto moisture along with the dust and pollen it is designed to catch. A filter that has gone a year or more without being changed is a common contributor to a sour or musty smell, and it is one of the easier things to check yourself. It is usually behind the glove box or under the dash on the passenger side, and swapping it is inexpensive compared to most other work on a car.A blocked drain makes everything worseThe evaporator drain tube routes condensation out from under the car, usually near the firewall. If you have never noticed a small drip of water under your car after running the AC on a hot day, the tube may be clogged with debris, and the water that should be draining out is instead sitting against the coil. That standing water turns a minor film of mold into a persistent, re-growing problem no amount of spray cleaner will fully solve, since the fix for a clogged drain is clearing the tube, not masking the smell at the vents.Carpet, padding, and floor mats hold their own moistureWet floor mats, a door seal that lets rain in during a sudden afternoon storm, or just months of humid air soaking into the padding under the carpet can all grow mold independently of anything happening in the AC system. This kind of smell is usually less sharp and more earthy, and it tends to get worse with the windows up and no airflow rather than spiking when the AC first turns on. If you can smell it more strongly down low, near the floor, the carpet and padding are the more likely source.A soaked carpet is harder to dry out than it looks, because the padding underneath holds water long after the surface feels dry. Pulling the carpet back to let both sides air out, or using fans and a dehumidifier inside the closed cabin for a day, does more than running the heater for twenty minutes.What actually fixes itReplace the cabin air filter