No heat condition, but a hot heater core!

[Brian Larson]

I am having an issue with the 91 200q I just picked up.  And it is cold here
right now which complicates the issue (10F or less in the mornings).

When I got the car, the heat worked fine.  Then the heat stopped one morning
last week and has not worked since.  The blower seems to work correctly, and
does not blow until the car is warm, and the air diverts fine between the
dash, vents, and floor.  The heater valve seems to be working correctly, and
the heater core is hot (both hoses are warm) in front of the windshield.

Is there a likely culprit for this, hopefully one that can be fixed without
pulling the car too far apart?

It might be a coincidence, but the climate control blinked the outside temp
light at me last week one day, but now this is not flashing either.

Please help,
Brian