It's a coolant level sensor. It causes the LED to blink when the coolant drops below a certain level. 

  It will also blink if you don't have enough coolant in the coolant/water mix.

  And they typically fail such that they won't blink at all if the thing cracks allowing coolant up inside the plug where the contacts are.... or sometimes it fails such that the light blinks all the time...

  Bill

[text/html]