this engine is in my step van freightliner mt45
But I don't know where to connect this loose cable. I made a short video explaining the problem

 

That loose wire for the fan clutch electromagnet usually plugs into the fan solenoid harness near the front timing cover or radiator side harness, depending on which MT45 setup you have. From the screenshot it looks like someone may have previously bypassed or repaired part of the wiring, so I’d be careful before connecting it randomly. The best thing would be to trace the wire colors against a wiring diagram or look for a matching unplugged connector nearby. If the fan clutch isn’t engaging properly, that loose lead is very likely the cause.