I'm not familiar with your particular car, but I can offer some general information that maybe will help.
First of all the infamous monovalve problem is with the 123 series cars from about '81 to ''85. I expect that yours has a different monovalve that is much more reliable. My '88 124 car has the newer style that seems to be much more robust.
The monovalve can most likely be found near the battery. It will have a heater hose going in and another going out. There will most likely be a two wire connector attached. If it is like the 124 car, you can disconnect this connector and it will open the heater flow wide open. If you disconnect this connector and the system starts making hot air again, you probably have a control problem, not a monovalve problem.
I don't know how knowledgable you are, so don't be insulted if I tell you some basics that you may already know. If I were about to freeze I would probably be in a panic to get information.
Make sure that the coolant is not low in the radiator. If so, is the temperature guage showing that the engine is coming up to operating temperature? If not, it could be that you need to replace the thermostat. I have had to replace a failed MB thermostat a couple of times over the years for this reason. Is the heater fan blowing in manual mode? In the cars of the late eighties, the climate control problems were more often in the climate control pushbutton unit than anywhere else. As I said I'm not completely familiar with your particular model.
Give us as much more information as you can, and we will all do our best to help.
Best of luck, and bundle up well,