Foul odor in car after being parked

Checked AC vents can't smell anything in them, turn AC on and car is okay. Smell comes after car has been parked. Drain is clear and system puts out water, tried spraying lysol into air intake at windshield does not seem to help. Have gone around the car using the old nose can't find source (carpets,seats etc.). Smell is sour/ or like dead animal. Any common AC problems to Blazer (2003). As an aside, how do you remove the interior plastic covers over the wheel wells on a 2dr Blazer do they just pull out?

Sounds like something crawled up into your A/C,Heater ducts and died.