Go to this web site.
http://www.geocities.com/chevroletc1070/wiringdias.html It's the wiring diagrams for our trucks in the "8 cylinder engine compartment" link, look for the 20DG wire coming from a octagon shape at the left front of the engine marked temp switch. It will go to the aft center connector for the firewall plug.
Now go to the "Instrument panel and bulkhead connector (part 1 of 2)" on the bottom left of the schematic is the bulkhead connector marked with an "A". On the upper "C" connector on the left side center connector you will see the 20 DG wire coming out and going all the way to the top right corner of the page.
Now go to "Instrument panel and bulkhead connector (part 2 of 2)" , and look at the top left and find the 20 DG wire again. Follow it to the ignition switch and then back out the same connector on the switch again as 20 DG. It will go to the "Cluster connector" mid page to the second from the top, left side connector. Looking to the right will show a "legend" of where each of those connectors goes to (the 4th one down says "temp ga lp". This is the connector on the circuit board on the back of your instrument cluster.
This is the proper wiring for the temp gauge. The reason it goes thru the ignition sw is so that it doesn't stay "hot" all the time. The gauge will only work when the ignition switch is on. It sounds like you have miswired the "Cluster connector" going to the circuit board.
Check the "Cluster connector on the (page 2 of 2) of the wiring diagram and make sure you have everything hooked up correctly.
If your still having problem, let me know, maybe I can help more. That is if this helped at all.