All gauges quit working. Replaced all stepper motors. Still nothing is working.

The sending units for whatever gage, sends signal to pcm, the pcm then sends serial data to instrument cluster, tells the instrument cluster what to do.
Can you check for instrument cluster trouble codes, if there are any? Some of the national brand auto stores will check codes for free. Sometimes they won't check for codes if the check engine lite isn't on, Just tell them it has been going on and off, they should check it.
Check voltage and ground for the instrument cluster. The ground wire for instrument cluster is black, the voltage circuits for instrument cluster are fuse protected.
bbbind.com
should have wiring diagram for instrument cluster. It will show the ground and fuses. There could be something wrong with your instrument or wiring circuit, somewhere. I can't properly diagnose this from where I am.

Is there power (B+) an ground's for instrument cluster ? When you turn the key to the run position ( not start ) do the gauges do the sweep test ?
Did you perform the Instrument Cluster Diagnostic System Check?
Go to Step 2
Go to Diagnostic System Check - Instrument Cluster
2
Turn the ignition ON, with the engine OFF.
Observe the instrument cluster.
Does the instrument cluster perform the displays test?
Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems
Go to Step 3

Turn the ignition OFF.
Disconnect the IPC.
Turn the ignition ON, with the engine OFF.
Connect a test lamp between the ignition 1 voltage circuit and a good ground.
Does the test lamp illuminate?

Perform Diagnostic System Check - Instrument Cluster , Diagnostic System Check - Driver Information Systems or Diagnostic System Check - Audible Warnings before using the Symptom Tables in order to verify that all of the following are true:
• There are no DTCs set.
• The control modules can communicate via the serial data link.
Review the system operation in order to familiarize yourself with the system functions. Refer to the following:
• Instrument Cluster Description and Operation
• Indicator/Warning Message Description and Operation
• Driver Information Center (DIC) Description and Operation
• Audible Warnings Description and Operation
If you don't know about serial data communication an DTC'S I strongly suggest you take your vehicle to a Chevy dealer or an ASE certified repair shop .