I k ow theres a thread about this but I just wanted to post my findings today. All summer whenever i turned the ac on i would get a must smell for approximately 20 to 30 seconds and then it disappeared, figured since 2004 the old girl had some dirt buildup on the evaporator since some engineer felt it wasnt necessary to have cabin filters on a super dity ehich is designed to be on a job site, oil field etc. After reading all the threads of fellow members pulling the blower box apart and finding the evaporator clogged solid with pollen,dirt,leaves I gave it a shot.....and I guess it's a good thing my box was SPOTLESS not even a dead bug in there, just a little surface dust nothing more, i guess me not using the heat nor ac that much plus shutting the system off whenever i go off payment or pass a farmers field who's being plowed payed off all these years?
So now where is the must smell coming from???? I think it could possible be the foam ford uses on the damper doors when they close to seal differnt vents? Soni grabbed some Simple Green and soaked everything I could spray, let it sit for a few minutes then hit it again, took the garden hose and rinsed the box all out, sprayed the evaporator just to be sure and then got a clean lint free rag and took simple green/warm water and I gentle washed the foam on the damper doors the best I could getting my hands in there to A-hold the damper open B-get the rag in there to wash the foam. Put the old girl all back together and started the A/C and all you get now is SIMPLE GREEN perfume LOL after she dries out completely I'll check the system for that funky must smell, hopefully i got rid of it? If not i seen on YouTube where people spray lysol spray into the cowl inlet with the blower on high to kill smells or bacteria possible in the system....well see how it goes though

 

The musty smell comes from the condensate drain. Make sure that when you use your AC that when you turn it off you get a puddle of water under the truck. AC systems remove moisture from the air and it goes in a pan under the condenser. If you have standing water in there it will stink. There is a deodorizer you can buy and spray in the system to make it smell fresh.

 

I flushed that all out when I had it apart, what I cant figure out is where on earth the water is suppose to start at then drain down to that drain opening? There is no holes leading to that drain, I had my scope in there and couldn't find the drain pan either? So I sprayed simple green in the drain hole and plugged it with a rubber bladder I have for hvac service, waited a few minutes and did that again then flushed it out with fresh water a few times.

and yes when i shut the truck off i have water that puddles under the passenger leaf spring

 

This is what I use. Do it yearly. AC always smells fresh.