I have a 2000 GT also, my light came on after the switch failed. My level was full and the light wouldn't clear even after cleaning the float switch, etc. Ford conveniently molded the switch into the coolant tank, so it has to be replaced with the switch.
Unless you want to fork over $100 for the tank/switch, you have 2 choices...
(1) Unplug the connector on the bottom of the tank which will turn the light out
(2) Live with the "battery" light as my wife refered to it as... LOL
I missed the coolant adding part... does your level drop, or are you just adding a little each time?