Antifreeze will be leaking in the floorboard on the passenger side, there will be a smell of antifreeze through the vents.
If antifreeze is filtering through your ac vents, other toxins could also be coming through. Don't take chances; see a mechanic.
Most likely the heater core is leaking. Has to be replaced.
yup, sign of a bad heater core
The "antifreeze smell out the AC vents" would mean only one thing, you have a small leak in the heater core.
the heater core may be plugged up
Your heater core has failed.
The heater coil probably has a leak causing steam and vaporized antifreeze to be released into the vents. Is it using coolant?
Your heater core is leaking, Need to REPLACE HEATER CORE.
Sounds like a freeze plug corroded or blew out; at worst, it might be a cracked block.
Check for a line leak to your heater core or a leak in the heater core it self. Usually it is under the dash on passinger side.