Ford: 1988 Mercury Topaz, mercury topaz, rest water

Question
I have been having trouble with my 1988Mercury Topaz (1988Ford Tempo) over-heating.  I don't know if it is the radiator the the coolent tank or if it is just because my Grampaw keep over filling the resivore.  He will fill it with water to the FULL HOT line. My friend said to drain the tank and the radiator and just add 1/2 water and 1/2 coolent to the radiator and the FULL CODL line in the tank.
So I need to know if that would work or should i do somthing else.

Thanks CHURCH  

Answer
What i would
do first is find out what is the cause of the over heating problem  is befoer i do  anything else.When you flush out your cooling sys. you put in 2 gal.of coolant then the rest water to much coolant will cause your hoses to crack.the first thing to ck is make sure the cooling fan is working & the way to do this is run the car and when it gets to operating temp. ck to see if the fan comes on.if not then turn on the ac with car runing the fan should come on after a few sec. If not let me know and i'll walk you through a step by step test.