Turn the key to "on" & cycle the temp slider from full cold to full hot in one move - listen for the servo motor in the dash making any noise. If quiet, I agree it's likely you have an issue with the motor.
Another thing to consider is the potentiometer that adjusts temp, which is on the back of the HVAC box. Mine is wonky... it keeps the valve closed until I get to full hot (85°+) and then fully opens it, so I effectively have a heat on or off function, no blending. I'm no electrical expert by any means, but I'd think it's possible your potentiometer isn't working, so it's not actuating the motor at any point. If that did happen to be your issue, there's a thread detailing how to go about fixing it by re-soldering the 3 joints on the back.
My heater was possessed. But after a simple exorcism, all is well. I've had this problem with my heater on my '94 FZJ80 for some time. I would select a temperature with the slider and randomly the heater would go to full heat. I could jiggle the temperature slider and it would return to...
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