It's not always the heater core, it could be the heater valve or the axillary water pump near the heater valve. Some times those two could leak, and the blower motor just sucks in the coolant smell from the engine bay. I smell the coolant too sometimes inside the car when i turn on the heat. but the smell goes away after 5 seconds.
1990 M5 Euro
You wash your car like it was your firstborn child, you tend to its needs like it was your own body, you protect it like it's your family, then you drive it like you stole it