The problem with the Ford solution to the musty odor in using YN-29 (combination of an acrylic resin and powerful antimicrobial formula eliminates) maybe the method used in applying it (or the tech applying it) and hence why it works for some (like me) and not for others (Riggo on FFH forum).
Also wab, it's not a lifetime cure. It will "wear off" (protects for 2-3 years is what Ford says). Mold growing in the HVAC box and on the evaporator core is a common problem across manufacturers. The problem is moisture remaining in the box. Get rid of the moisture and no problem. So, running the blower or heater (as drdiesel suggests) after A/C use for a few minutes should "dry out" the box.
Here's an old VW TSB (around 2006) on the issue:
Condensation is a normal characteristic in all A/C systems. If odor is noticeable without engine started, this may be due to mold and mildew in the passenger compartment, not from the A/C system.
All other possible sources of unpleasant odor must be addressed before performing the following procedure.
Consider the following conditions before proceeding:
• Evaporator housing drain must be open and free of debris.
• Cabin must be sealed with no water ingress into passenger compartment.
• Plenum drains must be clear and free of debris.
• Pollen filter must be clean and free of moisture.
Contaminants are effectively neutralized using the Aircomatic® II Ultrasonic Climate System Cleaner (VAS 6189A) and Airco-Clean® Ultrasonic Air Conditioning Treatment (VAS 6189/1). All other cleaning methods are no longer permitted.
• This procedure will be allowed only once per vehicle within the first 12 months of service.
• There must be an original customer complaint of specified odor symptom on repair order for this Technical Bulletin to apply. Adding on this procedure is not permitted.
• Subsequent applications, should they become necessary, will be customer's responsibility.
• Replacement of pollen filter under warranty is not allowed within the performance of this procedure. The pollen filter is covered only where specified at the appropriate maintenance interval.