Recently discovered a horrible smell in the cab of our Excursion? Any ideas? on 2001 Ford Excursion

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Nothing smells under the hood and no obvious animal carcasses on the chasis
(1) Answer
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Assuming you already checked under the seats and floor mats for food, etc. Does it happen when the blower motor is on or off? Or both?
It happens when just sitting there running or not it doesn't matter. We have left it sitting with the windows open for days and the odor is still there
Does the smell have the characteristics of a dead rodent? Is there the possibility of someone pranking you with a foul substance in the air vents in the passenger compartment?