Search Ebay for "electric fan switch". There are several kits for less than 20 bucks, with free shipping. It includes a coolant sensing switch that turns on at 185 degrees and back off at 175, a circuit breaker, a relay, and wiring, connectors, pipe fittings, etc.
It's an almost idiot-proof way to turn a cooling fan on and off, unless you're a particularly persistent idiot! If you wire the relay to a constant-hot circuit, it will even fix the flooding problems associated with hot soak after the engine is shut down on a summer day.
Jerry
