To find the relevant fuses, you'll need to consult your owner's manual. It will have a fuse box diagram showing the fuse locations and what each fuse protects. Look for fuses related to "radio," "power," "accessory," or possibly even the specific name of your radio system if it has one. There are usually two fuse boxes in a Chevy Avalanche – one under the hood and one inside the vehicle, often under the dashboard.
Without your owner's manual, providing the exact location is impossible.