Trouble with Isizu Trooper Big Horn Heater

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1995 Isizu Trooper Bighorn 3.0 import. Running excellent with one problem. Belongs to my Son who is freezing as the heating has never worked since he bought this 2 years or so ago. Am wondering if anyone can advise on chain of things to check to see what the fault might be. I bought a "service" manual on e-bay that wasn;t ! and tried to get my hands on a haynes manual but they don't do one so am looking to see if i can get some good advice. When heater control is selected ( electronic with or without A/C , I can hear the various slots etc moving/adjusting. Pipes are all warm .So my guess is some sensor is gone. Anyone what they are doing with these things ? replies welcome... Dad of freezing son!