If turning on the HEATER works to cool your car down, then yuor problem is cooling. Replace the rad.
If turning on the AC system works to cool your car down, then your problem is cooling fans. When the AC is on, your electric fans both turn on whenever your AC compressor is on. With AC off, run your car until it starts to heat up, or getting ready to overheat and check to see if your electric fan is ON. If not, this is your problem. Then you have to diagnose why the fan isnt working. This is only if you have elec fans on this car.