Can't locate where a plug goes
 

Can someone please tell me where this plug goes too? Its coupled up with a coolant temp sensor wire. Cars running a bit rough and we want to get this plugged in to see if that's the cause but we can't find where it goes (cars just had the headgasket done), it's stuttering abit on cold start but fires straight away after it's been running for about 10-15 minutes still idles rough but doesn't cut out, it does have 3/4 a tank of fuel left in there, we want to find where this plug goes first before we drain that out (it's been in there since august). Car currently idles at about 500rpm. I've connected my accessport to it today and nothing obvious seems to be wrong on there. *Edit* car is a 2007 mazdaspeed3
https://cimg4.ibsrv.net/gimg/www.maz...0d1accc7d2.jpg
https://cimg2.ibsrv.net/gimg/www.maz...f30261f23c.jpg