Radiator cooling fan

I have a 1985 Corvette, automatic, with 20,000 miles. The radiator colling fan does not come on. The IP temperature digital readout gauge works fine I have replaced the fan motor and fan relay. The fan does come on when the engine code checker is plugged in. My guess is the problem is with cooling temperature switch. However I can not find it on the engine. Can you tell me where it is located? Is there anything else which may be causing this problem? Thanks Tom