How can one differentiate a leaking rear heater o-ring from a leaking
rear heater core?

My rear heater emits the "essence de glycol" whenever the fan is run.
There is no visible leak from either the core or valve (nearby floor &
carpet is dry, etc etc).

Hmmmm.... I see that the o-ring is a $0.69 part and that the core is
a $69 part.  I suppose Murphy would say that is a *substantial* clue to
which part has failed.  Nevertheless I'd be interested in any comments
as to the mode of failure of each of these components.  (Also any leads
on the toxicity of glycol vapor... I was aware of the not pouring the
stuff on my pancakes, nor of leaving it about where pets or California
Condors might find it... but I hadn't realized that the vapor was toxic
in low concentrations.)

regards,
Ed
'84 plain-jane