Coolant temp gauge problem

1994redram said

The gauge housing is grounded. The harness has a dedicated ground wire that attaches to the cluster with a screw. The voltage regulator is grounded to the same screw. The regulator is putting out a solid 5 volts. The fuel gauge itself doesn't have a ground at the cluster.

I used a wire wheel and cleaned the block, firewall, frame and also cleaned all of the wiring terminals. I removed the sending unit and cleaned it and the block. I ran a wire from the battery ground to the body of the sender just in case, still nothing.

Ill try to switch the coolant temp wire with the fuel gauge wire and see what happens.

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I don't like to argue so just the facts... A typical 2 post gauge is not chassis grounded. If you remove one from the housing you will find the housing has a cut away area beneath the gauge. Only current path through a 2 post gauge is post to post.
A 3 post gauge is chassis grounded in the inst' housing. It even has a piece of metal on the back to help ensure this ground path. That current path is for only the mechanical limiter. You stated that you had replaced the mechanical limiter in the fuel gauge with a solid-state regulator.
You should have put a piece of tape on the back of that fuel gauge to isolate it from the chassis ground. The solid state regulator would be grounded somewhere else. We don't know that the remnants of the mechanical limiter is not wicking away a good portion of the solid state regulators output. 5.3 volts is one thing. Barely over 1 amp is another.
My best guess is a bad temp gauge that will not respond to high resistance/low heat. Do this... Use ohm meter to check the 2 gauges post to post. The fuel gauge should be 20 to 21 ohms. The temp gauge might be same 20 ohm gauge but could be a 13 ohm gauge too. The 13 ohm gauges are known to show low to nothing with low amps.