Welcome to the Forum from NH.
Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.
I hope that you will become a participating member in the Forum's discussions.
Pics of the truck, please.
Go to GMPartsDirect.com, enter your VIN, go to the Body section, and look for the exploded diagram of rear electrical harness. You should find what you are looking for along with the correct part number. And, the cost will be significantly cheaper than the local dealer.