Speedometer, gas gauge and instrument lights stopped working but radio and tach work fine. defroster fan runs on high and can not be turned off because no buttons on the dash work. car runs fine.

Sounds like you blew a fuse in the fuse box should tell which fuse is what on most covers Good Luck Mike

More then likely it's the stepper mortors inside the instrument cluster . If it were a ground issue more then just speedometer an tach wouldn't be working . Like instrument lighting an rest of gauges . GM had bad stepper motors inside instrument clusters , 2003 to 2006 . Watch this video
https://www.youtube.com/watch?v=oTm2oDx-_GM&t=30s

There may be a problem with the connection or the wiring harness between the instrument cluster and the body computer.
No Bus means no signal or power on the line between the two computers.

Best practice is to leave pinpoint testing of the instrument cluster & (Delicate) P/C board to an expert .....

If you need further help, reach me via phone at https://www.6ya.com/expert/ralph_2f0886b7d6af32ce

Hi, this is a pretty common problem on these Silverado instrument clusters. We can definitely help you out with that. Click here to learn more about these Silverado instrument cluster problems: Dr Speedometer Instrument Cluster Repair Speedometer Repair Guage...

The car sounds like it got flooded with water, do you have a title to it? Does it say salvage? If not then the dash issue has nothing to do with the other non electrical issues. There maybe a BCM (body control module) that is bad. Sounds like there are wires that are not connected to the thermostat to run the fan or the fan relay is bad or unplugged. The entire cooling system needs to be pressure tested. The coolant should be 50/50 water and coolant, there is a lot of things going on with the car so how many miles are on it and has the ignition switch been replaced?

The speed sensor on your trans sends a signal to the electronics telling it when to shift, this will be where you fault is.

When the gauges quit, does the ABS light come on? Does the gas gauge and temp gauge quit too?

IF YOU REMOVE INSTRUMENT CLUSTER AND UNPLUG IT,PLUG IT BACK IN AND UNPLUG IT AGAIN..DO THIS A FEW TIMES..IT SHOULD CLEAN OFF SLIGHT OXIDATION ON CONNECTOR TERMINALS..HAVE SEEN THIS BEFORE..I ALSO THINK REPAIR CONNECTORS AVAILABLE FROM DEALER

Stepper motor that controls the gauge !
How to repair GM Instrument Cluster Speedometer Gauge Stepper
You can buy the step motors off amazon to do all the gauges for $27. bucks .
Amazon com Dr Roc GM Instrument Cluster Gauge Stepper Motor Repair Kit X27...