not sure what engine you have, but you need a temp gauge. the yg wire does not look factory. the temp sender is usually in the housing below the thermostat, sometimes a second sensor is above the thermostat too. And/or a sensor is placed on the top of the head at the front where the water channel is which leads to your cabin heater. Probably some fiddlin has gone on with pvo.
Perhaps the easiest for now is an aftermarket temp sensor, there are many to choose from. I use engine guard watchdog, it bolts directly to the head, very accurate, easy to install, love it. I use it in conjuction with the ome gauge but the ome gauges can tend to fail or the earths oxidise/corrode over the 30+ years.