I am not a truck guy, but here is some ideas and questions.
1. Look on the back side of the data plate for a part number.
2. Maybe the Air Con/Heater is a MWO. A Modification to the truck. Is there a MWO data plate on the truck. If so, you then look up the MWO documents and the parts needed are listed in it.
What TM did you use to look up the old part #? What is the old part #? Did it have a NSN? When you look it up, sometimes there is an upgraded part # or NSN.
How old is the TM? and are there any changes to it? If you are looking at a TM that is too old, or it has not had all the changes to it, then that may be part of the problem.
