You have a leaking heater core. You might smell antifreeze if you turn on the defroster.
If the heater core in your vehicle goes out, you will not be able to use your heater or defroster. You will also notice the smell of antifreeze throughout the vehicle and possibly a film on the windshield from the antifreeze.
If the heater has a sweet smell it could be that the heater core is leaking engine coolant. When the leak gets worse you will have a wet floor on the passenger side and the windshield will fog up when you turn on the defroster.
A small amount of antifreeze smell may be emitted from the radiator overflow. However if you smell antifreeze you need to make sure there isn't a leak. It may be something simple if you catch it early.
It's either stopped up (no heat) or leaking. If leaking you will smell antifreeze (sweet sickening smell) and it might fog up your windshield with the defroster on. Leaking alot, the carpet is wet. Keep an eye on the antifreeze level.
If you smell antifreeze in the passenger compartment of your car your heater core is probably leaking. It's under the dash somewhere ( depends on the make and model ). Another symptom of a leaking heater core is moisture build up on the inside of the windshield when you run your defroster.
If you mean "why", then the answer is that there is a leak in the heater core.
I believe when you turn on the rear window defroster the heated mirrors also turn on
Most likely your heater core is leaking.
your vehicle may smell like antifreeze while driving because it is overheating
Antifreeze is used to heat the cab of your vehicle. When you turn on your heat in your cab, it opens up a valve that allows the hot antifreeze to flow from the radiator into the heater core. If there is a leak in the rubber tubes that flow into the heater core, or if the heatercore is damaged, you will smell the antifreeze in the cab of the vehicle. You may need new hoses to the heater core and/or a new heater core.
with a push button directly under the defroster control