This can be a very difficult problem to get rid of; the mold tends to make spores that plant themselves in the interior of the car and it tends to keep recurring anytime mioisture enters the car to allow it to sprout.
During used car prep in our shop, we have found that for us the best procedure to get rid of musty odors (usually mold related) in a vehicle is to remove the seats and carpeting, and use a commercial upholstery cleaning machine and a pressure washer to thoroughly clean these items; then allow them to air dry out in the sun on a warm day (this takes a very long time usually). At night after the vehicle is reassembled, we take a floor fan and direct the airflow through the vehicle with the windows open and allow it to run all night, to further dry the interior.
There are various commercially available fungicides on teh market; anything that kills mold in your bathroom will probably do so on your vehicle interior as well. Bleach works well at killing mildews.
The downs side is that many of these products may also damage the appearance of your car/s upholstery; I would suggest getting a few different bathroom cleaner products and testing them on inconspicuous places, such as the bottom of the seats where you can't easily see the spot, to check for color damage.
Or, you may want to take the vehicle in to a commercial car detailing shop to have it professionally cleaned and shampooed (about $100 in most areas). If so, getting the interior of the vehicle absolutely dry afterwards is most critical to prevent it from returning.
I hope this is helpful; if so, an accept would be most appreciated! thanks!