Do you get the same smell if the controls are in recirc? If so, it could be accumulation of crap on the heater core from lack of air flow over it. If it is actual exhaust you would smell it all the time that the HVAC was on and not in recirc. It wouldn't matter if the A/C was on or the heat.

 

When recirc is on, no smell. When I put HVAC to blow only on the floor, recirc automatically disables itself. So when heating up inside the truck, only floor, no recirc= exhaust smell

 

Are you positive that its an exhaust smell, or just a nasty smelling heat?

 

The recirc door covers the only opening for air entering the HVAC case. After entering the HVAC case, the same air is either ducted to the heater core or the A/C evaporator. If you are getting exhaust smell from outside the cabin, it should smell whether the heat is on or the A/C is on.

 

If you are getting an actual exhaust smell it is highly likely that your OEM exhaust manifold is cracked and leaking.

 

Yeah... What @Skizzo said...

 

After driving around multiple times today, I tested different things... and really, it just stinks like ass. I swear it was a lot like exhaust, but today non-circ with heat on, it stunk. Bad.

I sprayed some Febreeze into the vents beneath the wiper blades and it made a drastic improvement. I fear something crawled in and died, and it's stinking up my cab.

 

You probably have mold that started on the a/c evaporator and was caused by a plugged up condensate drain. The correct fix would require at least disassembly and chemically cleaning the HVAC system. Maybe after making sure the drain is clear you could remove the blower assembly and if you see or smell anything spray a can of KMP Coil Klean (or equal) at any suitable target inside.
On a high mileage 2000 Prizm we owned I actually used a hole saw to cut holes in plastic case to vacuum out about 1/2 gallon of sunflower seeds some rodent had placed inside. Sealed the 1.5" holes w/aluminum foil tape & the everything worked perfectly for another 50,000 miles.
By the way anywhere in Arizona can get very cold especially the mountains which can exceed 12,000'. With the normally cloud free winter skies it gets cold as soon as the sun is gone. You can be sure the heater and A/C were used equally.