get a 10mm socket wrench(if you dont know what that is find someone who does)the fill bottle under the engine covers,use the 10mm to unbolt it,slide it up and put it on the latch that the Hood
close on to(its the black metal that actually laches the Hood
,the hoses that is attached to the bottle,would lengthen out for it to reach)it would slide over the latch and stay inplace.start the car,and
take off the cover on the bottle,and start filling.it is going to take a good while to full up,cause air is going to be in the system.you are going to notice that bubbles would start coming up from one side of the tubes. and dont overfill it.buy a little more coolant than you need,cause you are going to waste some.when you think its full(at this time the fans might be on)excess coolant might be bubbling up out of the bottle,you can control that by putting on the cover and closing it and opening it.it depends on how much its closed or open would dertermin the behaviour of the coolant in the hoses coming and going from the bottle.you are going to notice,that after awhile you would be able to notice the way the water is flowing in the bottle,by moving the cap back and fourth,when you reach that point you are done,close the cap(make sure it has the right level)put the bottle back.
btw.we had a gts with the same problem,even changed the radiator,same thing,it wasnt until we experimented and came up with what i just said,that the problem went away.