There are wires going to the top of the coolant resoviour(the one with the
radiater cap).The plug at the end of the wires , attaches to the resoviour
bottle.It may either be disconnected , or have corrosion on the
plug/socket.If no wires are to be found, perhaps the plug has fallen down
below the resoviour. With a flash light look between the bottle and the
tail light.This disconnected plug is a sure fire way to get your
symptoms.Perhaps someonewas poking around the engine compartment just
before the light went on,and bumped into the plug.