You might have a heater core leaking. My friend tells me mine leaks everytime he gets in my truck. He says it smells like burnt antifreeze. Everybody else that rides in it says they smell something burning like smoke or rubber or plastic.
I never can smell anything. Apparently they don’t have to leak very much to impart the smell into the airflow because I haven’t had to top up the radiator for several months, and I never see any leakage on the underside of the heater box.