I apologize, I did miss the part with the 12 volt system. I would assume the other motor {6 volt} ran at twice the speed a {getting 12 volts} and probably burned up in the process. Did the 53 have the resistors in the plenum box. I wonder when the other motor went out if it burned up the resistors. denny is correct about the voltages, if you had a volt meter it would make things easier, you could actually see what voltage you are receiving at the motor.
