HelloI would disconnect the electrical connection to the heater control valve ( as having the valve connected will screw with the readings) and put a voltmeter onto it. Turn the ignition on and turn the heater knob to hot. The voltage should be zero or at least very low . Turn the control to cold and the voltage should go up to 12v. It could be the other way round, but moving the control knob from hot to cold should change the voltage. If this works, it must be a faulty control valve. New ones are known to fail. If there is no change in voltage, then it must be either the control panel behing the control knob, or the power feeding to that.