If you want OEM part only it is "BEHR 202 820 73 10" - just google that and you should find several offered for less than $100.

From the MB SLK forum site:

Here is a procedure to check things out.
Pull the connector after dropping the panel in the passenger footwell (2 screws)
With the car off there should be full voltage red to brown (grnd).
With the car on if there is full voltage white/pink control panel is defective.
With the car on and fan off on control panel there should be no voltage between the white/pink and brown. As the fan speed is increased the voltage should increase as the fan switch is turned higher.
If the fan runs all the time the regulator has failed closed so there is full power to the fan.