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