Why would your neon overheat while in a park but when you turn on the heating inside of the car it would cool down to a normal running temperature?

There are a few possibilities as to the cause, bad thermostat, low coolant level, inefficient water pump, restricted / clogged radiator, and so on... The reason it cools down, is the heater unit in the passenger compartment is actually a small radiator, and when you warm up the interior, you are cooling the coolant, so to speak. Also, a lot of the newer vehicles will run the electric cooling fans when the heater / defroster is on, and this will cool the car down if it is overheating due to the cooling fans not being turned "on" by the coolant temp switch. To check if it the fan circuit, let it idle in your driveway, and when the temp starts climbing, the electric fan(s) should turn on when the coolant temp is about 225 deg. Fahrenheit. If they don't turn on then, put the heater control to defrost, and then, if the fan(s) come on, you know where to start looking...