I was going to put the 250 ohm resistor in series with the manual switch so that I don't put a dead short across the sensor. I didn't intend on altering the resistance of the sensor at all. Manual switch for emergencies only - after I replace the original sensor.

Meanwhile The red sensor I bought will go into my W190 2.6. This car is a U.K. version. There is no auxiliary sensor at all in this car. The Auxiliary fan only operates off the air con. Maybe the auxiliary fans were not needed in the U.K. due to ambient weather there. - certainly required in Australia - in particular where I live - summer temperatures often above 40 degrees C (over 100 degrees f)