Musty smell from AC vents
Needs to be cleaned. Many products on the market to do just that. Try looking at WalMart (and others) in the ‘automotive’ section for Ozium.
Replace the cabin air filter
It might take care of some or all of the problem
Next time at the shop good idea to ask them to make sure the AC drain isn’t clogged up.
Ozium is a very good product, and it is often used in mortuaries because of its ability to mask the odor of rotting flesh.
However, Ozium will not remediate an underlying problem related to mold growth or other sources of obnoxious odors in an HVAC system.
I think that the OP would likely be better-served by using either Frigifresh or Lysol spray on the intake of the car’s HVAC system.
The cabin filter is upstream of the A/C evaporator, so you need to remove the filter before spraying in something to get rid of the mold. Remove cabin filter and set A/C to MAX and spray something into the intake where the filter was. Lysol works OK. I like baby powder scent. Wurth makes a product specifically for this problem that works very well. Other products are mentioned in this thread. Hydrogen peroxide works great but is corrosive. Use hydrogen peroxide only as a last resort.
Seconding making sure the drain is unplugged. You can stick a wire up the hose, but you must be very careful not to prick the evaporator.
Also stop using recirculate or max air so much, us it only to get a quick cool down and then go back to outside air.