I had this happen to a corvette I had covered and closed up outside only much worse than you describe. Mold spores are everywhere you can’t eliminate them. They need a conducive environment to grow. That’s how they got going in your closed up car.
I had good results using chlorine dioxide gas to kill off the active growth. First you have to stop the leaks then go after the mold. Ask what gas they used. Did they run the fans to circulate through ducting? How long left exposed to gas? Etc.
You can buy the gas producing packets and kit online. Follow the directions to a T.