If my heater in my truck was running fine and then starts blowing cold air, is it my thermostat or my heater coil?

What is the temp gauge reading? Feel the heater hoses, are they hot in both or only one? Backflush the heater core, Ford Rangers and Dodge Dakota's are featured in some posts and a back flush worked every time.

I have a rheem air and heat unit. The system will not come on even after adjusting temp to get it to come on. Just small click is only noise there is no kick on it will not blow. HELP!