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someone wetting themselves in the car because they couldn't get to a toilet quick enough
I am not a expert with this stuff but try putting those smell things by the vents in your house and spray the rooms with smell stuff about every hour.Again im not a expert with this so if it dosent work im very sorry
You do not list the year. Unless you are talking about a Fiesta built in Europe that is fairly new, it does not have a Cabin Filter. The smell you are smelling is more than likely mold or mildew growing inside the vents. Very hard to remove. Search Google with the term "Odor from dash vents" for advice on how to remove it.
there could be a dead animal in your vent
There may be a person that has a reason to retaliate with you!!! They put urine in the air intake of the blower motor. Depending on type of vehicle, it may be cheaper to junk and get new car then replace dash. Don't mess with people that handle your food and vehicle!
you need a head casket
This smell indicates there is a fuel leak methinks, get it looked at by a garage!
Any smell from the car vents is unrelated to the AC system unless it is an antiseptic smell. Generally, the smells are environmental from your engine compartment, where the fresh air is taken from. If you smell something burning from the vents, then something is heating up to much under your hood. Check your compressor for a worn clutch, or oil leaking onto the exhaust manifold.
Smell may be (heated) mouse dropping and urine. We found a mouse nest in the (and partially made from the) air filter. Open the air filter and check, mouse was still hiding down the air hose. This search was done in an attempt to find that sour smell.
They have perfume running through the vents. Wash your clothes to get the smell out or it'll smell like that FOR A LONG TIME.
dryer sheets in the heater vents may help
Unless someone peeded into your inlet vents, next guess is are you a smoker??