I have a 2008 Pontiac G6 and when the heat or defrost is on I can smell antifreeze inside the car. Why is that?
More than likely the heater core is leaking.
Agreed. In cars I’ve had with a leaking heater core, the floor on the passenger side would be wet. If you have ethylene glycol based coolant, you’ll want to get this addressed sooner rather than later.
Nearly always there is a leak in the heater core. The leak may be small enough that you don’t see moisture on the floor.
I’ve had several of these; in a 1971 Mercury Comet, a 1984 Chevy Impala, and a 1976 Ford Granada. The one in the Ford was difficult to fix since the whole car seemed to be built around the heater core. The Impala was the easiest to fix.