With the advent of automotive HVAC automated temperature control, auto manufacturers have adopted an "always hot" heater core. This allows instantaneous addition of heated air to the HVAC system to maintain a constant interior temperature. Unfortunately, if the door actuator fails, it is buried in the dash board. Failure of the door controller (which is about $90.00 ) requires the removal of the dash board (R&R = 10 hours or $800). Fortunately, older technology is available to correct this for a fraction of the cost.
For eighty years, the way heater temperatures were maintained was by throttling the amount of hot coolant from the engine. This method is still applicable today. By cutting the heater core engine heat source hose and inserting a fairly standard heater core throttle, one can control heat entering the HVAC system the "old way". The valve is available at most auto parts stores (see image). For about $30.00 , one now has a working HVAC system!