Water leak

OK. The only other time I have noticed a stench that strong is when the AC drain hose was blocked and either leaking on the floor or inside somehow. AC drain should be located passenger side, very close to floor, under blower motor. It’s really just a rubber tube that can get clogged on old trucks or disintegrate. It drains from inside cab AC unit to the inside of hood to the ground. Typically mold wouldn’t survive on plastic and metal. I think your AC drain line is clogged.

 

It could be dead mice.

 

Once a year I spray a whole can of lylsol into every vent with truck shut off then go home the next day open windows and crank fans for a bit to get rid of lylsol smell quicker .. it works wonders imo ps I meant a can for whole truck lol

 

Give this a try. Used it on a boat, it eliminates every odor possible and kills mold, mildew, insects. You release the gas, close it up and leave it. Then after so long you run your hvac to kill anything in ductwork. Reset Chlorine Dioxide | Safely remove odor, mold, mildew, algae