Air conditioner drain line stopped up or windshield seal leak
that covers 99% of the reasons
If its a sweet smell you may have a heater core leak and the fan is blowing antifreeze thru the vents. If its a moldy rotten smell you have mold growing in your vents. This happens when the moisture does not drain properly causing mold. You can purchase a spray at most auto parts store to kill and stop this and refresh your air vent system.
heater core, tough job.take it to a qualified auto technician
change the cabin filters
Depends on the smell. What does it smell like?
heater core
you need a new heater core
Perhaps a dead mouse.
Have the cabin air filter changed.
Most of the time it is caused by a hole in the heater core, do you smell a sweet smell? The sweet smell would be the antifreeze.
Your house may smell like electrical fire or nail polish remover is coming out of the AC vents because you have fried wiring. You should make sure that you do not have a wall fire.
sounds like your air conditioner compressor is bad.
bad heater core bad heater core