One wire should be bigger and it wires direct to hot(usually the starter solenoid). The other two wires, one wires direct to hot the other to the switch. The wire to the switch should be wired to go cold on off and hot on run. Crank the engine and then turn off the key(on a diesel with manual pump). Test the alternator for voltage output and if the alternator is still putting out voltage, switch the two smaller wires. On an engine with electronic ignition the engine will not die with the spade wires reversed.
