The first step is to check that the ventilation blower is working. It is a small blower motor tucked in the dash board under the right vent that always runs whenever the ignition is on. Always.

Cut a piece of paper 1cm x 1cm, turn ignition on (doesn't matter what button is pushed in on climate control unit), and hold paper up to temp sensor grill near overhead light. It should stay up there.

If it falls off, ventilation blower is not blowing, and not pulling air past the temperature sensor up in the overhead light assembly. The CCU doesn't get accurate info of how hot the interior is, so blows full blast. It never slows down.

I've had this problem in two 124 cars. Other people have reported that the foam hose from the temp sensor to the vent blower has disintegrated.

It's something to start with.