I smell anti- freeze! Several posters suggested the freeze out plugs on the exhaust side. But, with the engine running at temp, any liquid is evaporated. The plugs are rusty, but I don't see any moisture around them. Does that mean that I am safe?

Anti-freeze if "Evaporated" leaves visible residue (White-ish).
IF it were seeping from the "Freeze plugs" and then Evaporating You'd see It.

I Hate to even suggest it...BUT Heater Core ?

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'84 300SD sold
124.128

When mine was leaking -- This is what it looked like



You can see the coolant leak is very obvious. If yours does not look this way then you are not leaking here. The place that was causing coolant smells to get through my vents was the hose that sits at the very back of the engine on the right side that leads up towards the fuse box into the cabin. The hose had cracked and was leaking coolant. It would get into my vents and cause the smell.

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Adding the appropriate dye to the coolant will also make it easier to find.