1989 Chevrolet S 10 Chevy S10 Heat Works Great Switch Defrost There Burning

the smell is a leaking heater core.

there may be something in the duct either burning from the heat restricting the flow of air.

Roy

Thanks Roy,
I went to the auto parts store to get a new heater core, the service man totally confused me so I want your oponion please. He seems to think it is the Air blend door actuator because the heat works fine with no smells it only smells when the defrost is on, the defrost does blow hot air but not much to defrost the windshield. I realize the air blend actuator is located near the heater core so it may be worth it to replace them both. I also have to apologize I told her it was a 1989 and it is a 1998. sorry had a blonde moment!
Thanks for your help.