You need to connect the fan through a relay powered from the battery or starter solenoid through a fuse. Then control the relay by a temperature switch to ground and powered from an ignition 12 volt source.
this is a good diagram I used an adjustable temp switch available at any auto parts store for 20-30 bucks. this will allow you to adjust the fan turn on temp. there is also a way to put a bypas switch in but i never bothered and i have been doing well with my electric setup. if you have ac you probably should add ht bypass just in case.David, you can use a standard automotive relay from most auto parts stores. The coil or control circuit of the relay should be connected to a key-on 12 volt source so the fan will not run after the key is turned off. You will use 4 terminals of the relay. Terminal 30 will be wired to a 25 or 30 amp fuse from the battery, starter solenoid, or horn relay 12 volt distribution point. Terminal 87 will go to the fan. Terminal 85 will go to the key-on 12 volt source. Terminal 86 will be wired to the coolent temp switch in the manifold.
Here is a diagram. Disregard the wire colors.
If you have A/C, you can wire a trinary switch to turn the fan on when the condensor needs more airflow. If you don't have a trinary switch in the A/C system, you can use another relay to control the fan relay from the compressor clutch circuit.