[Gelbster]

Here is an interesting ,related thread for older P-Cars that gives the basic elements of the discussion
Ballast resistor/oil & AC fan low speed resistor - "redesigned" - Rennlist Discussion Forums
http://rennlist.com/forums/996-forum/857242-cooling-fan-resistor-3.html
http://www.arcolresistors.com/resistors/hs100/
https://www.amazon.com/uxcell%C2%AE-Aluminum-Housed-Wirewound-Resistor/dp/B0087ZCZ9E
A thermo switch seems like a good addition?