I think your '66 is similar in operation to my '68. Look at the dash trim, the lower lever is "temp". It will have one cable to do the work and is attached to the door marked as "temperature door" at the far right of the box. The upper lever operates two cables and are marked "defrost" and "heater". The upper lever first engages the defrost cable which is attached to the door marked as "defroster cable", to the far left in the box. As the top lever continues its travel it engages a second cable that operates the third door marked as "shut off door" which is in the middle of the box by the two heater core inlet tubes. Kind of "wordy", but I hope this helps.
