Hi @devman867 ,
It is a process of elimination to find every problem that you have with the radio/radio speakers
First test the individual “faulty” speakers with an Ohmmeter (function found in a DMM (digital multimeter) for continuity to prove that they are all OK electrically. Also inspect the wiring connections from the speaker drivers to their terminal blocks to ensure that there are no loose connections or dry solder joints. You’ll have to work out how to gain access to the speakers themselves or it may be easier to try the the test below first, regarding testing the wiring between the radio and the speakers. If you can’t “see” the speakers with an Ohmmeter then there is a break somewhere in the wiring which may be on the speakers themselves
Here’s a link that shows the wire functions of a 2010 GMC Sierra radio harness. It should be OK as it is in the range 2008-2013 which I think are all the same but I may be wrong about that. If in doubt search for (insert year model) GMC Sierra radio wiring diagram.
Here’s a link to a video that shows how to remove the radio from the dash so that you can access the wiring harness connector located at the back of the radio.
If all the speakers test OK that eliminates the speakers as the problem then you can use the Ohmmeter to prove continuity in the wiring between the radio harness connector pins (unplug the connector) and the various speakers.
If that also tests OK it eliminates the wiring as the cause of the problem then you may have to prove that the various radio speaker outputs are OK by connecting a test speaker directly to the radio output wires for ones that don’t work correctly or at all.
After that the problem is in the radio itself. It will have to be opened and the problem further diagnosed