I had a pack rat, my truck reeks like rat **** and poop whenever I have the vents on after the engine heats up...and only when i'm moving slowly or stopped.. My initial thought is "oh god it got into the heater core or blower motor"

But, upon some research it seems like the air enters the HVAC unit from the upper passenger dash, and that the air enters this unit from the cowl; between the windshield and hood. If this is the case, it seems like a large size rat would not be able to access any of the HVAC system.

To further support this, I have realized the air only smells bad if i am idling, or if I am driving less than about 40mph. This tells me that when moving slow or stopped, its just sucking in ambient reek-air from the under hood area, and when i am moving at speed, the fresh air is present and flooding the HVAC.

So, can anybody tell me where the outside air comes in to the HVAC? is it the cowl area?

 

yes it is the cowl at the base of the windshield, when not in recirculation mode. I am not sure where it comes from when recirculation is turned on.

 

cool thanks, thats good news. it stands to reason that an animal of any significant size would not be able to enter the blower motor area.