Thanks fellows' ,you are all a wealth of information and I honored to receive your help. My heater is indeed a 100238 dealer installed unit as per your info. The drivers side smaller defrost duct is missing from somewhere in the past. I can also see that the "fresh air heater" has better designed ducts that fit the slots and screw holes. If any one has a 212915 left side duct and the s clips #2123978 for the first style unit or the new and improved fresh air heater defroster ducts #212674 & 212675, I would be interested in buying them. Again, many thanks to you all. Don