It is most likely from mold in and/or under your upholstery.
Unplug clogged evaporator drain plug.
"The musty smell of the attic filled my nostrils" BWHAHAHAHAHAHAH
The first step to getting a musty smell out of a closed up house is to open the windows. You will also want to burn some candles. Spraying Lysol will also help get rid of the musty smell.
The musty smell that Brian notices when he wakes up is caused by the dampness and mold growth in the shelter. The combination of moisture and lack of ventilation can create a musty odor.
Bleach
Vinegar
wash everything and febreze...
you wash it????
try using an air freshener
A "musty" smell would describe a damp animal
It can be a sign of mildew or mold.
Even when your vents are closed, air still is being pulled into the car. The musty smell means you inside cabin air filter needs to be changed. You can gain access to the filter through your glove box.