I'd need to see a schematic however if this follows past MB and other brands conventions, the following will apply. This isn't a resistor, it creates a variable voltage electronically.

On many systems, the thin wire receives a variable voltage from the heater / AC controls. Low voltage is slow fan and higher is fast. Monitor the thin wires voltage to ground, it is probably dropping out causing the fan to slow.

To go any farther we need a schematic / test procedure for this car.