I don't think you have a fresh air intake to the heater on that? any car will have fresh air and recirculate functions to prevent fogging up the inside windows. They'll turn on the AC with the defrost on also. In the north we don't always understand all that's required in other climates, because I think there has been one time in my life I had to turn on the defrost to clear the inside of a windshield. Your backhoe probably doesn't have vents aimed at the windshield either. Sometimes there was a mounted fan you could aim at the windshield, but that still needed some warm dry air inside to work. Without a fresh air intake, opening a window is the best you can do.