back up lights and gear indicator not working

Have ALL fuses tested to start with... That's 'tested' with a test light or volt meter.

The gear indicator (PRNDL) screen is a common failure on all 03-06 Silverados and is caused by problems inside the gauge cluster. Often it's due to cracked solder joints but can be other components inside the gauge cluster as well. The good news is it's definitely repairable and isn't too expensive. I've personally fixed many hundreds of them with this issue over the years. If you'd like to learn more about the problem and how to get it fixed please visit our site at www.drspeedometer.com Thanks, Fred Dr.Speedometer