This would cause the resistor temperature to rise to a point where it will melt and possibly ignite the plastic blower housing to which it is mounted.

The evaporator utilizes a power resistor to regulate blower speed. Should a short occur in the "low or medium speed" circuit while the blower is in high speed, the short will cause the supply voltage to feedback through the resistor to ground, not drawing enough current to trip the fuse.

What to do: Book it in at your next service. There is no deadline and no charge.