86 supra MK II no heat
 

The automatic climate control hasn't worked in years, was stuck on high heat which was fine. Not I get little or no heat and winter is Coming. Already changed the thermstat and topped up the rad but no change. recently installed an inline block heater. Heard could be air trapped in the heating core??? I think maybe the hot/cold mixing valve has shifted toward cold. Anyone know where that door is and how to reposition it? Any other ideas would also be appreciated.