Jwithing
Senior Member
This is going to sound weird... but check your truck water/mold and mildew. We had a similar problem with a work vehicle last year. Employees started reporting a dead animal smell that slowly got worst and worst. The garage tore the thing apart and never found a cause. Eventually we discovered leaky rear passenger door. The sub-floors were soaked and the vehicle had slowly developed a horrible mold issue over a few months. True Story! Good luck.
^ This madee think of the notorious, leaky 3rd brake light. Don't discount other areas because it seems stronger with the air on. Your nose could be tricking you by thinking that the smell is stronger with air on when you could just be getting a breath of fresh air from your vents, then another punch in the nostrils from the stank source.