On my 84 I had just taken asome copper pipe and soldered an "L" shape, just so that it would fit in place of the monovalve. I used a rasp to make the ends rough so the hose wouldnt slip off and used some hose clamps to hold it in, and it worked fine. Basicaly what your doing is making the heater on HOT mode all the time, and the only way to modulate heater temp is to turn the fan down or up. You just wont get the functionality of the blue-white-red dial. Im not sure if your ac will work with this mod, my ac didnt work anyway. But it will be good to get some hot air flowing until you can afford a ripoff repair kit

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1981 300SD 512k OM603