Over heat

I have a 1991 Audi 100. The car's fan's have two phases but they only turn on once the red over heat lamp in the car turns on, I was wondering if anyone knows what would cause this to happen; what controls the temperature. Also, if anyone knows the location of radiator fan motor resistor.

Thank you.

if fan dosn't run in high ck thermo switch located botom right of radiator ohm it out should turn on at 190-200 deg F if ok ck relay # 7 botom left in axsilery box under drivers steering wheel panel most likely thermo switch resistor may be integrated in fan not sure but can ck