To all the good and knowledgeable guys in this forum:

Thank you, thank you, thank you! Whoever, wherever you are...

I have a 1981 300 SD that either blow hot or cold air, regardless of the eigine rpm or how hot the coolant temperaturewas, for the past 6 months. Several folks in the forum suggested that air temperature sensor in the cabin could be the culpit. I followed their instruction and found the "foam tube" connecting the sensor and the black suction tube had broke off due to its age. I then used a 9" length of water pipe insulator (5/8" ID) to replace the original foam tube (6" long). This insulator tube is relatively firm so that pushing it single-handedly for the connection will not be too difficult. Wala! problem solved. No more toaster/freezer hybrid car as my carpool budy jokingly put it . You guys must know the sweet feeling when an age-old and potentially expensive problem finally release its grip on you.