Using the A/C with fresh air produces a musty smell of the air entering the cabin from the HVAC system. I looked in the manual if there was an air filter for the cabin to change with no avail. After returning from an overseas military assignment where the truck was in storage for 3 years, took it for service and asked the dealership to troubleshoot and solve the problem. There was no filter to change/replace so the service department flushed the fresh air intake (at the base of the windshield with a pressure washer/cleaner) and also ensured there was no clog for the A/C drain. Now the fresh air and A/C does not have the musty smell.
I tried spraying Lysol into the fresh air intake with fan going on high ,no help tried 50% water and Clorox neither helped at all . And since to get to the problem you had to remove the AC freon drain the antifreeze to get the heater and AC core out I thought of using my leaf blower . The one air vent that put out the most air on driver side by door I taped the two on right ,pluged the heater outlets . I took off the trim around the instruments and plugged the one in the center . With the grill off the far left I got a three inch flex hose hooked up to my leaf blower I had to hold it there no way to connect it and ran leaf blower for about 10 minutes . It must have had a bunch of stuff on there and it blew it off probably still in there but at least not on the AC evaporator coil . I saw where nothing cam out but at least the smell seems to be gone for now.
It has smelled like this for years . My wife says she don't want to ride in it now smells so bad . To get to the evaporator core where the problem is . You practicly have to dismantle the cab you got one bolt behind the brain box,the toe side panels there is a bolt they also have the door step threshold needs to come off . the panell below the steering wheel needs to come off to loosen the steering wheel. I just have not done it yet. I shrayed 50-50 clorox & water in there wiyh fan running on high . It smelled better at first but next day it smelled the same .