Don't overlook the monovalve. The late 123 cars are known for the heater control valve diaphragm rupturing causing heat in the heater core overcoming the cold evaporator air.
In case you're not familiar with the monovalve, it is on the firewall next to the battery with heater hoses in and out. Take the four screws out of the top and lift it up to see if the diaphragm is ruptured. If so, you can get a repair kit from the parts shop or your dealer for around $30.