its a "freeze plug" on the bottom of the thermostat housing. More than likely you just need a new O Ring. Do a search for Viton o ring on here and you will find the size.
'14 Tuxedo Black ST3
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'04 CD Silver SVT, #877, 1 of 135 May 2011 ROTM, FF11: Best Display, Best of Show; AFN12: Best SVT, Best Audio, Finest Five, Carlisle Nifty Fifty