it's the simplest circuit that will do what you want.
The key part(s) is the R2 potentiometer. It's a 3watt RS part of 25 ohms variable. This will set the trigger point to any point around 200 - 250 ohms from the Mercedes 4 term sensor. The diode across the relay prevents spikes from wiping the transistor. Any NPN transistor with a TO-220 case will work.
The graph shows the relay pull-in point (12V relays typically pull-in at about 8V or greater) based on the resistance of the CTS. Note that at 225 ohm point (adjustable by R2), the relay will fire and start the fan.
It's not elegant but it'll work. A 'better' solution involves use of an OP-AMP and transistor plus more parts.
PS: A really neat solution is to forget the relay and use a "heavy enough" transistor and fire the fan directly. In this version, the fan's speed will be proportionally controlled and come on gradually as the resistor changes in the area of 250 to 200 ohms. Cool!