I want to install a trailer hitch so is there a wiring harness somewhere

MAYBE bring vehicle or VIN# to dealer and check what 'options' your car had. It may be 'pre-wired' for an accessory kit to be added easily.

Etrailer.com has some excellent videos on installing trailer hitches and wiring for many makes and models, including a Traverse. I purchased mine from them. Service and instructional vehicles were excellent.