Is it necessarily the case that I will smell antifreeze if there is a problem with the heater core?

All vacuum lines and pods are in place and functioning.

With regards to the heater box door is anyone aware of the internal workings of the HVAC black box? Is there such a thing as a heater box door? No mechanic I have spoken to knows what to tell me.

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1984 300 SD 384,000 Miles