Mazda Repair: Overheating, diaphram, water pump

Question
My Mazda 1996 Protege 1.5 Liter Overheats. We recently changed the water pump and the thermostat. When the heater is on it doesn't overheat as much. What should we check next? We were thinking we needed to replace the radiator, is that necessary? Any new insight would be appreciated.

Answer
first check the termastat itself by removing it and placing in it hot water you should see the diaphram in it move if it doesn't it needs to be replaced if the rad is holding fluid it shouldn't need to be replaced unless the cooling fins are missing or badly damaged. also make sure the fan it coming on if not before replacing the fan check the thermastat,fuses.and wining to make sure there of good repair