Water temperature gauge wiring questions

[bryantreams]

Ok folks first of all I hate to start a new thread to ask this but I've searched and searched and I'm still not clear on how to wire up the temp gauge.

I have a sensor in my intake that has a coil covered wire coming out and going to the dash which is what I thought went to the temp gauge but that doesn't seem to be the case. I have read that I'm supposed to have a sending unit in one of the heads with a wire going to the gauge however both heads are just plugged. I went and bought a duralast temperature switch (part number TU5) and plan on installing that when I install a new water pump (mine has play in the bearings), thermostat, and heater core (heater core was disconnected by p.o. and coolant in floor pad so I assume it's leaking) tomorrow but I have no idea what wire goes to it or how it connects. It has a little round prong that I've never seen before. This truck is new to me and the wiring harness in the engine bay is a mess.

I have read that you can test the gauge by disconnecting or grounding the temperature switch wire but I didnt want to jump in and start grounding random loose wires. And yes I plan on cleaning up all the random wiring, just trying to figure out if I want to replace the entire wiring harness or just clean up what I have.