What is the cause, and how can we remove the musty odor that is emitted from the air conditioning system. The car is only driven once a week. We do not use any tobacco products in the car.
I would start by checking the condition of the cabin air filter. If that doesn’t take care of it you probably have bacterial growth or mold around your evaporator. Some shops can send an anti-bacterial/mold chemical through your system and that can help. You should also periodically run your vent, bringing in fresh air, with the ac off to help dry your system out, heat with no ac is best.
Check the cabin air filter.
Remove cabin filter, then replace cabin filter cover, then put fan on high and spray Lysol into the air inlet at base of the windshield as you vary the inside temperature from hot to cold. Alternatively, on some cars it works better to spray the lysol into the open cabin filter housing. That should kill the mildew in the air conditioner.
I prefer the baby powder scent Lysol for this job because no one objects to that odor.
Wurth makes a citrus-smelling product that is made for this specific purpose. It seems to last a little longer than Lysol, but costs a lot more.