Electrical issues with automobiles can be very complex.   The illustration above is a connector for a wiring harness to the speed control resistor for the heater and air conditioning.   One terminal on that connector is black.   It is black because of high resistance at the connection.   The resistance caused an interruption in the flow of the electrons causing an electrical arc.   Over time the current could not flow beyond this point and the fan blower became inoperative.   The heat caused by the electrical arc can get hot enough to melt the plastic housing of the connector.   It could even cause an electrical fire. Resistance can be caused by a poor connection or corrosion.