Update: Well, I ordered a 12v fan motor awhile ago and I'm starting to put the heater unit back together. My original 6v motors have a case diameter of about 2.5". The new 12v motor is a fat 3" and does not fit inside the round heater core. So now I'm searching for a 12v motor that is 2.75" in diameter or less. Or, I'm thinking about using my old 6v motor with a voltage reducer. I'd rather find a 12v motor. I need two - one for the main fan and one for the defrost fan on top. The search goes on.
