ECT and interior temp gauge?

I started a thread about a month back about my truck high idling. Well now I've noticed the temp gauge fluctuating. Into the bad zone - hot. Anyway, I just purchased a new thermostat and ECT sensor which I will change out on the weekend. Will flush out the radiator too, to make sure there is circulation.

Question: Where is the sensor that reads the temperature which corresponds to my interior temp gauge? Does ECT do this aswell or is it another one? Where is it located?

I can see my ECT is behind my timing belt cover downstream of upper radiator hose. Will change that out on weekend.

I've been studying the ttora fsm site and cannot find what I am looking for. Any information/circuit diagrams on the temperature system would be appreciated.

PS: Last night when slowing down and putting my foot on the clutch the engine started reving, up and down up and down. Not spluttery bad idle, more like someone was reving the throttle.