01 E320 Cooling Fan

[DslBnz]

Thanks in advance for taking the time to help me with my plight.

My mother has a 2001 E320 that has recently developed a curious condition. Upon starting the car, and about 10 seconds have passed, the electric cooling fan will slowly whir to life until it reaches maximum speed. For as long as the car is running, hot or cold, AC on or off, the fan will continue to power. Humming noisily as it goes.

My questions are:
What could be the culprit causing this problem?
In what area of the 320 would I find this part?