X2 on the heater core or heater core hose.
It could also be a coolant leak somewhere else, and the smell is being pulled into the cab through the climate system air inlet (the one located lust below the windshield).
If your windows fog up (which is a safety issue), then I'd guess you have a heater core problem. I had a Jeep Cherokee with a blown heater core. THe windows would fog up so bad that I couldn't see out the windows.
Keep an eye on the coolant level.
I hope it's an easy fix! Keep us posted.