Drivers side of the cylinder head above the intake and exhaust clear at the rear.
Mystery solved! Never occurred to me to look there. I'll examine when I get home. I'm glad it's out back, the wire on front runs close to belts and it's an inconvenient place to run a neat wire harness.
Up till mid 90's then they did away with that one and went to just using the sensor mounted in the Tstat housing and having the ECU tell the gauge what to do.
That makes total sense when you already have the complexity in place (eg. the computer). No fun for home-made though.