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.
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 .