mk3sickness

So i picked up this car recently and just want to fix a few qwerks before i sell it. I know my way around a car pretty well though i'm 100% new to both nissan/datsun and the 240z itself. my temp gauge isn't working so first thing i want to check was the wiring to make sure the sensor has proper power and a good ground, this car however through me for a loop because i see what seems to be 2 possible sensors. i was hoping if anyone could help me identify which one is the temp sender and what the heck the other is. i believe the one circle in green is the temp and the one blue is my mystery. and how do i test/ replace a temp sender assuming im right on which one is which. red is area in question. Thanks for the help in advance