This is a first, I've never heard this complaint about a 124 before. Everyone always complains the aircon doesn't work, and/or the heater works too well!

I'll second the monovalve recommendation. The monovalve is normally open - power is applied to close it and block the flow of coolant to the heater core. When something fails, normally the monovalve stays open and cooks the occupants of the vehicle. I suppose it could have stuck closed, just never heard of this failure mode before.

South Africa is right hand drive, correct? On left hand drive cars the monovalve is between the two firewalls, in front of the battery. I'm not familiar with the underhood layout of right hand drive versions of the 124, so it may or may not be in the same location.

- JimY