Could be many things. Can't say exactly what. But, here are a few things I would check. Do you have trees in the area you park your car? If so, you could have leaves pine needle or other dibree in you heater vent that could have caught fire from your heater motor. I actually had this happen once, it filled my pickup cab with wood smoke. It smelled like that every time I used the heater until I sold the truck. You heater core could have a leak and the antifreeze smell could be coming through your vents. Or your heater motor itself could be going bad and melting the winding on the motor armiture. I'm sure there could be other things like electrical shorts, lost cigar behind the ashtray. Hope that gives some help.
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.
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