If you can put voltage on the fan and it spins, pull the wire off the coolant switch and connect it to the body somewhere while the engine is running. If the fan turns on, it's the thermo switch. If it doesn't, it's the relay. The thermo switch is a ground switch. It grounds the circuit when it's 1) wet, 2) really hot. If your coolant level is below the switch, it will not turn on. There are 2 thermo switches. One for the gauge, one for the fan. The little one with one wire is for the fan.

If you overheat once, usually the thermostat is ****ed. It leaks the mercury out when it overheats and never opens enough when it's supposed to after that.

Fill your coolant system with the heat on full blast or you'll trap air in the heater core. Use the bleeder screw to burp all the air out and refill until ALL the air is gone or else Boyle's Law will never take affect. English: If your coolant system doesn't hold 15 PSI, you need to fix that problem before you're going to see any change in your overheating situation. Water under pressure is harder to boil.