What would a detail shop use to clean the intake plenum or A/C coils?x2 on the detail shop - if you are willing to pay. Not only do they know how - they have the equipment - like steam cleaners and such.
Also mentioned but not specifically highlighted - the [email protected]@ smell is likely mold in the intake plenum or A/C coils. You need to spray something in the cowl with it running and cycle between full heat and full AC to get the cleaner on the proper coils. I have used a bleach water mix - but I am sure there are better things that don't have the risk of discoloring things.