Use a circuit tester and see which one goes hot when the brake is depressed.
On many vehicles that are using digital technology, tapping into a wire is now a no-no. Have to use a proper connector and interface. Can't do it on my Ford. Heck, even my 2015 Harley uses digital signaling, have to buy special switches with the proper wiring harness to attach anything to a wire. Don't know about your Chevy.
2017 Ford F-150 Crew Cab 4x4 3.5 EcoBoost
2014 Cruiser RV Fun Finder 215WKS
2015 Harley Road Glide Special in Amber Whiskey
2019 Mustang Bullitt
Yamaha Grizzly 660 (his)
Polaris Sportsman 500 H.O.(hers)