1989 Chevy C1500 Single cab (2WD) with 5.7 V8 and 700R4 trans

Been working on my truck and dealing with a bunch of stuff the previous owner did to it.

One of the items i want to fix is getting the coolant temperature sensor to report to the gauges.
Previous owner cut the wire and ran a new one to a gauge they screwed into the bezel. not only did it look bad, but it cracked the plastic and did not work.

so my question is this:
if i run a new wire from the coolant temp on the engine block, where do i splice it in to get its signal to the gauge?
I assume this would be done under the dash inside the truck, but i am not sure where it goes or what colour wire goes to the temp gauge.

I have read that the moon gauges can go bad and that might be a reason the previous owner replaced it with an external one. but this is my first step. I have a second gauge cluster for testing after this.

Thanks All!