if your carpets or upholstery gets wet and takes a long time to dry then mold can grow, but it is probably a plugged drain for the air conditioning inside the car, you can clean out the hose with a coat hanger or compressed air and it should be hanging down on the passengers side firewall.
It can be a sign of mildew or mold.
"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
you wash it????
Vinegar
wash everything and febreze...
try using an air freshener
A "musty" smell would describe a damp animal
hi smells like your wiper bottle might need new water in it ============================================================= Well, You might check the cabin air filter. If it gets wet, mold will grow on it giving out the musty smell
It is most likely from mold in and/or under your upholstery.